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: https://secure.phabricator.com/D10019