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