Migrate Audit comment text into new storage

Summary:
Ref T4896. This is substantially similar to D8196.

Migrate the comment text out of the `audit_comment` table and into the `audit_transaction_comment` table. Do double reads on `PhabricatorAuditComment` so the APIs aren't disturbed. The old table is still updated.

Test Plan:
  - Before applying migration, cleared cache and browsed around. Things looked fine, except no comment text.
  - Applied migration.
  - Cleared cache, browsed around, saw all my old comments.
  - Added some new comments.
  - Spot checked migrated and new rows in database.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10020