Add a "Draft" state for revisions, and action bucket support

Summary:
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.

Test Plan:

• 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).

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Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18625