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