Add a "Revision status" field to Herald for Differential revisions

Summary: See PHI280. We have a similar field for tasks already, this is generally a reasonable sort of thing to support, and the addition of "draft" states means there are some pretty reasonable use cases.

Test Plan:

• Wrote a status-based ("status is needs revision") Herald rule.
• Tested it against a "Needs Revision" revision (passed) and a "Changes Planned" revision (failed).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18938