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: https://secure.phabricator.com/D19657