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 PhabricatorAuditComments 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