R1:dc5c87f74cda

Hide Audit comment table reads behind an API

Summary: Ref T4896. Buries all direct access to the table so we can limit the surface area affected by the migration.

Test Plan:
- Grepped for `PhabricatorAuditComment`.
- Grepped for `audit_comment`.
- Viewed a bunch of comments.
- Added a comment.
- Reindexed a commit.
- Searched for unique term in new comment.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision…