Rename "PhabricatorAuditCommitStatusConstants" to "DiffusionCommitAuditStatus"; remove "MODERN_"
- This class is now a real object, so get rid of the "Constants" part of the name.
- Rename it for greater consistency with other modern objects.
- Get rid of the MODERN_ tag now that the old constants are gone.
Test Plan: Bunch of grep, browsed around.
Reviewed By: amckinley
Maniphest Tasks: T13197
Differential Revision: https://secure.phabricator.com/D19657