Add datasources to allow search revisions by project.

Summary:
When having lots of repos, seeing "all revisions in this project" is hard, and we ended up adding herald rules to basically copy project tags to the revisions on a per-project basis. Adding a "tagged: project" function to the Repositories search field allows users to find differentials within a project.

Fix T10850.

Test Plan: search differentials by tagging project and repository in the Repository field

Reviewers: avivey, epriestley, #blessed_reviewers

Reviewed By: avivey, epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T10850

Differential Revision: https://secure.phabricator.com/D16096