R1:3d78c0eff707

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…