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