Migrate Contracts to Solidity v0.6 (#2080)

* Initial migration to Solidity 0.6.x - v3.0 first steps (#2063)

* Initial migration, missing GSN, 721, 777 and Crowdsales.

* Add _beforeTokenOperation and _afterTokenOperation.

* Add documentation for hooks.

* Add hooks doc

* Add missing drafts

* Add back ERC721 with hooks

* Bring back ERC777

* Notes on hooks

* Bring back GSN

* Make functions virtual

* Make GSN overrides explicit

* Fix ERC20Pausable tests

* Remove virtual from some view functions

* Update linter

* Delete examples

* Remove unnecessary virtual

* Remove roles from Pausable

* Remove roles

* Remove users of roles

* Adapt ERC20 tests

* Fix ERC721 tests

* Add all ERC721 hooks

* Add ERC777 hooks

* Fix remaining tests

* Bump compiler version

* Move 721BurnableMock into mocks directory

* Remove _before hooks

* Fix tests

* Upgrade linter

* Put modifiers last

* Remove _beforeTokenApproval and _beforeOperatorApproval hooks

package.json

@@ -44,7 +44,7 @@
   },
   "homepage": "https://openzeppelin.com/contracts/",
   "devDependencies": {
-    "@openzeppelin/cli": "^2.5.3",
+    "@openzeppelin/cli": "^2.7.0-rc.2",
     "@openzeppelin/gsn-helpers": "^0.2.3",
     "@openzeppelin/gsn-provider": "^0.1.9",
     "@openzeppelin/test-environment": "^0.1.2",
@@ -63,7 +63,7 @@
     "micromatch": "^4.0.2",
     "mocha": "^7.0.1",
     "openzeppelin-docs-utils": "github:OpenZeppelin/docs-utils",
-    "solhint": "2.3.0",
+    "solhint": "^3.0.0-rc.2",
     "solidity-coverage": "github:rotcivegaf/solidity-coverage#5875f5b7bc74d447f3312c9c0e9fc7814b482477",
     "solidity-docgen": "^0.3.14"
   },