Give Audit an informational "This commit now requires (something)..." transaction

Summary: Ref T2393. This adds a state-change transaction hint to Audit, like we have in Differential. This is partly for consistency and partly to make it more clear what should happen next.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2393

Differential Revision: https://secure.phabricator.com/D17243