R1:a641ec82a323

Add an "Authority" control to Packages to support "Watcher" packages

Summary: See T13657. An install has "watcher" packages which should not allow owners to "Force Accept" other packages.

Test Plan:
- Created package A, which I own, on "/", with "Weak" authority.
- Created package B, which I do not own, on "/src".
- Created a revision which touches "/src" and added package B as a reviewer.
- Attempted to accept the revision...
- Before patch: permitted to "Force Accept" for package B.