R1:70ddb1c45fe4

Allow packages to be added as revision reviewers via the UI

Summary:
Ref T10939. This lets you add packages as reviewers manually.

"Project Reviewers" now lists both projects and packages. I have renamed this to "Coalition Reviewers" but that's probably horrible and confusing. I'm not sure "Group Reviewers" is much better.

Test Plan:
- Added a package as a reviewer manually.
- Joined it, got authority over it.
- Saw the review on my dashboard.
- Accepted the revision, got authority extended to…