R1:f965126dc454

Migrate audit comments to transactions

Summary:
Ref T4896. Depends on D10052. This is the major/scary migration, but not really so bad. It is substantially similar to D8210, but less complex because there are fewer actions here.

This moves `PhabricatorAuditComment` storage to `PhabricatorAuditTransaction`, then reads `PhabricatorAuditComment`s as a proxy around the new objects.

Test Plan:
- Before migrating, browsed around. Nothing appeared broken.
- Migrated cleanly.
- Viewed old transactions…