Modernize and clean up "PhabricatorAuditStatusConstants"

Summary:
Ref T13631. Move "PhabricatorAuditStatusConstants" to a more modern object ("PhabricatorAuditRequestStatus").

Expose the status value via Conduit.

Test Plan:

• Ran bin/audit delete.
• Viewed a commit with auditors in the web UI.
• Grepped for affected symbols.
• Called Conduit with the "auditors" attachment, saw auditor statuses.

Maniphest Tasks: T13631

Differential Revision: https://secure.phabricator.com/D21599