Differential - add DifferentialDraft to track whether revisions have draft feedback or not

Summary: ...do it somewhat generically, so we could fairly easily add this to other applications. Fixes T3496. I got a wee bit lazy and decided not to migrate existing drafts. My excuses aside from laziness are doing it this way will let us see if anyone complains, we can always do a migration later if people do complain, and there's likely to be a lot of garbage data for older / bigger installs, and the migration didn't seem worth itgiven it would also likely be expensive in these cases.

Test Plan: made a draft inline comment on DX and observed DX had a note icon on Differential home page. made a draft comment on DX and observed DX had a note icon on Differential home page. deleted a draft inline comment and noted icon disappeared from Differential homepage. Submitted a draft comment + inline comment and noted icon disappeared.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3496

Differential Revision: https://secure.phabricator.com/D8275