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 and auditors.
- Used "Add CCs".
- Added CCs with mentions.
Reviewers: btrahan, joshuaspence
Reviewed By: btrahan, joshuaspence
Subscribers: epriestley
Maniphest Tasks: T4896
Differential Revision: https://secure.phabricator.com/D10103
