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

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -1100,12 +1100,6 @@ final class DiffusionCommitController extends DiffusionController {
             'blue',
             pht('Closed'));
           break;
-        case PhabricatorAuditStatusConstants::CC:
-          $item->setIcon(
-            PHUIStatusItemView::ICON_INFO,
-            'dark',
-            pht('Subscribed'));
-          break;
         default:
           $item->setIcon(
             PHUIStatusItemView::ICON_QUESTION,