R1:ac2f5a10469a

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