Allow repositories to be associated with projects

Summary: Ref T4264. Ref T2628. Ref T3102. Allows you to associate repositories with projects. In the future, you'll be able to write Herald object rules against projects, use Herald fields like "Repository's projects", and search by project.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3102, T4264, T2628

Differential Revision: https://secure.phabricator.com/D7881