Introduce an AuditStatus object for commits and move some callsites to it

Summary:
Ref T13195. See PHI851. Add an object, analogous to the DifferentialRevisionStatus object, to handle audit status management.

This will primarily make it easier to swap storage over to strings later, but also cleans things up a bit.

Test Plan: Viewed audit/commit lists, saw sensible state icons. Ran bin/audit synchronize, got sensible output.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13195

Differential Revision: https://secure.phabricator.com/D19646