(cherry picked from commit 5c19876f)

* switch to using Context internally

* add context import

* Add smoke test to make sure enabling GSN support works

* Update test/GSN/ERC721GSNRecipientMock.test.js

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Upgrade truffle

* add missing awaits

* Revert "Upgrade truffle"

This reverts commit f9b0ba9019650fdbf8919a33e6fba834689e6b04.

> openzeppelin-solidity@2.3.0 lint /home/luiserebii/github/openzeppelin-solidity
> npm run lint:js && npm run lint:sol

> openzeppelin-solidity@2.3.0 lint:js /home/luiserebii/github/openzeppelin-solidity
> eslint .

> openzeppelin-solidity@2.3.0 lint:sol /home/luiserebii/github/openzeppelin-solidity
> solhint --max-warnings 0 "contracts/**/*.sol", alongside some minor grammatical corrections

* Add base Context contract

* Add GSNContext and tests

* Add RelayHub deployment to tests

* Add RelayProvider integration, complete GSNContext tests

* Switch dependency to openzeppelin-gsn-provider

* Add default txfee to provider

* Add basic signing recipient

* Sign more values

* Add comment clarifying RelayHub's msg.data

* Make context constructors internal

* Rename SigningRecipient to GSNRecipientSignedData

* Add ERC20Charge recipients

* Harcode RelayHub address into GSNContext

* Fix Solidity linter errors

* Run server from binary, use gsn-helpers to fund it

* Migrate to published @openzeppelin/gsn-helpers

* Silence false-positive compiler warning

* Use GSN helper assertions

* Rename meta-tx to gsn, take out of drafts

* Merge ERC20 charge recipients into a single one

* Rename GSNRecipients to Bouncers

* Add GSNBouncerUtils to decouple the bouncers from GSNRecipient

* Add _upgradeRelayHub

* Store RelayHub address using unstructored storage

* Add IRelayHub

* Add _withdrawDeposits to GSNRecipient

* Add relayHub version to recipient

* Make _acceptRelayedCall and _declineRelayedCall easier to use

* Rename GSNBouncerUtils to GSNBouncerBase, make it IRelayRecipient

* Improve GSNBouncerBase, make pre and post sender-protected and optional

* Fix GSNBouncerERC20Fee, add tests

* Add missing GSNBouncerSignature test

* Override transferFrom in __unstable__ERC20PrimaryAdmin

* Fix gsn dependencies in package.json

* Rhub address slot reduced by 1

* Rename relay hub changed event

* Use released gsn-provider

* Run relayer with short sleep of 1s instead of 100ms

* update package-lock.json

* clear circle cache

* use optimized gsn-provider

* update to latest @openzeppelin/gsn-provider

* replace with gsn dev provider

* remove relay server

* rename arguments in approveFunction

* fix GSNBouncerSignature test

* change gsn txfee

* initialize development provider only once

* update RelayHub interface

* adapt to new IRelayHub.withdraw

* update @openzeppelin/gsn-helpers

* update relayhub singleton address

* fix helper name

* set up gsn provider for coverage too

* lint

* Revert "set up gsn provider for coverage too"

This reverts commit 8a7b5be5f942002710cba148f249cb888ee8e373.

* remove unused code

* add gsn provider to coverage

* move truffle contract options back out

* increase gas limit for coverage

* remove unreachable code

* add more gas for GSNContext test

* fix test suite name

* rename GSNBouncerBase internal API

* remove onlyRelayHub modifier

* add explicit inheritance

* remove redundant event

* update name of bouncers error codes enums

* add basic docs page for gsn contracts

* make gsn directory all caps

* add changelog entry

* lint

* enable test run to fail in coverage

* GSN Advanced guide

* Tweaks

* Apply suggestions from code review

Changed word from Destoys to Destroys in line number 220.
https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol#L220

Need review. Thanks 👍

Fix typo

* replace readme badge

* remove travis config

* move to codecov

* add codecov config

* fix coverage running

* GSN basics guide (#1847)

* Fix linter error

* Initial draft for basic GSN guide

* Add CustomerAcquisitonCost link

* Review 'gas costs' phrasing

* Rename relay to relayer

* Apply suggestions from code review

Co-Authored-By: Santiago Palladino <spalladino@gmail.com>

* Improve transition to contract usage explanation

* Improve payment section phrasing

* Apply suggestions from code review

Co-Authored-By: Andrew B Coathup <28278242+abcoathup@users.noreply.github.com>

* Apply more review suggestions

* Add code

* Add unreleased warning

* GSN Docs Tweaks (#1860)

* Adjusts links

* Adjustments

* Update gsn.adoc

Fix typo

* Adjusts copy and links for GSN guide (#1865)

* Adjusts copy and links

* Formats links

* Adds website link

* remove attach_workspace

* leave default working_directory

* use npm ci for installing deps

* rename dependencies job

Fix typo

* Add circlci, remove .travis.yml. Fixes #1839.

* Fix circleci config

* Add slack notification

* Add circleci status

* Fix config.yml, we don't need separate truffle image and instances.

* Matches oz-sdk node version

* Seems node:10.13 doesn't work for CircleCI either

* Remove npm update in circleci

* CircleCI use package-lock.json as cache key and install only the lock version

* Add back travis.yml

* Rename job build to test

* CircleCI requires that job to be called build instead of test or build_and_test

* Fix the way to run jobs

* Fix CircleCI

* Change back to npm install

* Add workflow and split into tasks

* Add workspace

* Restore cache

* update readme circleci instructions

* make step name more accurate

* Imporvement: functions in SafeMath contract overloaded to accept custom error messages.

* CHANGELOG updated, custom error messages added to ERC20, ERC721 and ERC777 for subtraction related exceptions.

* SafeMath overloads for 'add' and 'mul' removed.

* Error messages modified.

* Update CHANGELOG.md

* added _safeTransferFrom function

* added safeMint functions

* added package-lock.json for consistency, don't know why it changes

* added initial suggestions/modifications

* change _safeTransferFrom to internal, reverted package-lock.json to original, and changed ERC721Pausable to override _transferFrom instead of transferFrom

* included tests for safeMint functions

* modified safeMint tests to be on ERC721Mock contract

* added safeMint to ERC721Mintable & respective test to ERC721MintBurn.behavior.js

* Rename package and repository name from docs and scripts

* undo root package rename

* add @openzeppelin/contracts as subpackage with release automation

* synchronize @openzeppelin/contracts version

* remove private field from package.json

* make file patterns absolute

* change wording of a comment

* use a saner version script

Add: Interface names should have a capital I prefix.

* add OwnableCautious with associated mock and tests

* extraneous comma removal and cleanup

* remove non-documentation changes from branch

Change URLs to openzeppelin.com

Update URLs and names to OpenZeppelin and OpenZeppelin Contracts

Update URL to openzeppelin.com

* update readme with new branding

* update readme text with new branding and links

* add newer link to api stability document

Squashed commit of the following:

commit fcf35eb806100de300bd9803ce3150dde1ecc424
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 17 17:16:04 2019 -0300

    remove all docsite dependency

commit eeaee9a9d43d70704f6ab17b5126ddbd52b93a50
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 17 17:15:23 2019 -0300

    update solidity-docgen

commit f021ff951829ea0c155186749819403c6b76e803
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 17 17:05:06 2019 -0300

    update docsite script for new setup

commit ff887699d381cfbbe3acf1f1c0de8e22b58480f3
Merge: c938aa1d 84f85a41
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 17 16:46:46 2019 -0300

    Merge branch 'master' into antora

commit c938aa1d9ed05ac83a34e2cebd8353f8331ad6d6
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Tue Jul 16 18:24:29 2019 -0300

    make component name shorter

commit 5bbd6931e02cbbd8864c82655ad0f390ceead5f3
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 10 20:16:17 2019 -0300

    add all info to docs templates

commit 39682c4515d7cf0f0368ed557f50d2709174208a
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 10 20:13:49 2019 -0300

    fix npm docsite script

commit 7ae46bd4a0437abf66150d54d05adf46e3de2cab
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 10 18:48:05 2019 -0300

    convert inline docs to asciidoc

commit cfdfd3dee4b4bf582fde22c8cb6e17a603d6e0c8
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 10 17:34:52 2019 -0300

    add missing contract names in readmes

commit 15b6a2f9bfb546cf1d3bf4f104278b118bf1b3f4
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 10 17:16:47 2019 -0300

    fix script path

commit 80d82b909f9460d1450d401f00b3f309da506b29
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 10 17:13:53 2019 -0300

    update version of solidity-docgen

commit a870b6c607b9c2d0012f8a60a4ed1a1c8b7e8ebd
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 10 17:03:53 2019 -0300

    add nav generation of api ref

commit 069cff4a25b83752650b54b86d85608c2f547e5e
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Wed Jul 10 16:32:14 2019 -0300

    initial migration to asciidoc and new docgen version

commit 55216eed0a6551da913c8d1da4b2a0d0d3faa1a8
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Tue Jun 25 20:39:35 2019 -0300

    add basic api doc example

commit 0cbe50ce2173b6d1d9a698329d91220f58822a53
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Tue Jun 25 19:31:31 2019 -0300

    add sidebars

commit 256fc942845307258ac9dc25aace48117fa10f79
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Tue Jun 25 15:22:38 2019 -0300

    add page titles

commit f4d0effa70e1fc0662729863e8ee72a8821bc458
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Tue Jun 25 15:19:41 2019 -0300

    add contracts index file

commit b73b06359979f7d933df7f2b283c50cb1c31b2a0
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Tue Jun 25 15:14:52 2019 -0300

    fix header levels

commit fb57d9b820f09a1b7c04eed1a205be0e45866cac
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Tue Jun 25 15:11:47 2019 -0300

    switch format to preferred asciidoctor format

commit 032181d8804137332c71534753929d080a31a71f
Author: Francisco Giordano <frangio.1@gmail.com>
Date:   Tue Jun 25 15:05:38 2019 -0300

    initialize antora component and convert docs to asciidoc