Allow auditors to be added and removed from commits in a modern way

Summary: Ref T10978. Ref T7676. Make auditors work more like reviewers, so they can be freely added or removed.

Test Plan:

• Interacted with auditors via "Edit Commit" and API.
• Comment area is still oldschool and doesn't work yet.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978, T7676

Differential Revision: https://secure.phabricator.com/D17181