Allow projects to review revisions

Summary:
Ref T1279. No actual logical changes, but:

• You can now add projects as reviewers from the revision view typeahead ("Add Reviewers" action).
• You can now add projects as reviewers from the revision detail typeahead.
• You can now add projects as reviewers from the CLI (#yoloswag).
• Generated commit messages now list project reviewers (Reviewers: #yoloswag).

I'll separate projects from users in the "Reviewers" tables in the next revision.

Test Plan:

• Added projects as reviewers using the web UI and CLI.
• Used arc amend --show --revision Dnnn to generate commit messages.
• Viewed revision with project reviewers in web UI.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7230