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 the package review.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10939

Differential Revision: https://secure.phabricator.com/D15914