Add a "Draft" state for revisions, and action bucket support
Ref T2543. There's no way to put revisions into this state yet, but start adding support for when there is.
Adds the status constant, plus support for bucketing them.
- Manually put a revision in "Draft" state by updating the database directly.
- Verified my drafts showed up in a "Drafts" section on the bucket view.
- Verified others' drafts did not appear on the action bucket view.
- Viewed revisions, queried for "Draft" revisions, etc (stuff we get for free).
Reviewed By: amckinley
Maniphest Tasks: T2543
Differential Revision: https://secure.phabricator.com/D18625