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).