Projects for DifferentialRevision

Summary: T2628, Adding project tags to revisions

Test Plan: Edit revision, verify projects can be tagged. Add project hashtag to comments or commit templates, verify revision is tagged with project

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9686