R1:dcd7a316d239

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…