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 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