[post-snap] Merge OUFR and react-next into react

[JustSlone]

(from @ecraig12345)

Part 1/4 in a series of changes that must happen in master after v7 development moves to a branch, but before beta publishing:

1. (this) [post-snap] Merge OUFR and react-next into react #14688 OUFR/react-next merge
2. Re-export date-time components from react #14679 Move date-time components
3. Move @uifabric packages under @fluentui scope #13384 uifabric/fluentui package renames
4. Set up beta releases #14689 set up beta releases

For version 8, we plan to merge office-ui-fabric-react (current code) and @fluentui/react-next (function component conversions and other breaking changes) into @fluentui/react. There will be no version 8 of office-ui-fabric-react, and no packages/office-ui-fabric-react in the master branch of the repo. (It will of course stay around in the 5/6/7 branches.)

The changes below may be done in multiple PRs to a staging branch.

• Merge OUFR into react
• Merge react-next into react (last chance to review FC changes)
• Delete OUFR
• Make react-next be a pure re-export package from react (and bump version to 9-alpha)
• Update gitignore so that removed files don't come back in a bad merge
• Update code references to OUFR or react-next (enough to build)

[ecraig12345]

Closing in favor of #14789