Accommodate project reviewers in Differential search

Summary:
Ref T1279. Two changes to the search/query for Differential:

• "Reviewers" now accepts users and projects.
• "Responsible Users" now includes revisions where a project you are a member of is a reviewer.

Test Plan:

• Searched for project reviewers.
• Verified that the dashboard now shows reviews which I'm only part of via project membership.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7231