R1:c7e7b63f155f

Rename "PhabricatorAuditCommitStatusConstants" to "DiffusionCommitAuditStatus"; remove "MODERN_"

Summary:
Depends on D19656. Ref T13197. See PHI851.

- 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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13197

Differential Revision…