R1:89b942c183eb

Move Audit to proper Subscriptions

Summary:
Ref T4896. Currently, subscriptions to commits are stored as auditors with a special "CC" type.

Instead, use normal subscriptions storage, reads and writes.

Test Plan:
- Ran migration and verified data still looked good.
- Viewed commits in UI and saw "subscribers".
- Saw "Automatically Subscribed", clicked Subscribe/Unsubscribe on a non-authored commit, saw subscriptions update.
- Pushed a commit through Herald rules and saw them trigger subscriptions…