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 (inlines, comments, accept/reject/etc, add auditors, add ccs, implicit CCs).
  - Added all of those comment types.
  - Edited a draft.
  - Deleted a draft.
  - Spot checked the database for sanity.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10055