Begin transitioning audits to modern (string) status constants, from legacy (integer) status constants

Summary:
Ref T13195. See PHI851. Audits currently have older integer status constants. We've moved almost all object types away from this to string constants (which are better in basically every way, and particularly way better for exposing over the API).

Commits/audits are currently accessible over the API and expose these constants via a "statuses" constraint.

Prepare to move toward modern string constants by defining a new, more modern map of status details and defining the existing methods in terms of it.

Test Plan: Browsed audits checking for icons/names/open-ness, saw no changes. This change should have no user-visible effects, as it just reorganizes code.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13195

Differential Revision: https://secure.phabricator.com/D19642