R1:5b74b8b765ad

Add basic "Subscriptions" application

Summary:
Basic infrastructure for generalizing subscriptions/CCs for T1808, T1514 and T1663.

- Implement `PhabricatorSubscribableInterface` and you'll get a subscribe/unsubscribe button for free.
- If there are any auto-subscribed users (like the question author) you can specify them; this makes more sense for Tasks and Revisions than Ponder probably, but maybe the author should be auto-subscribed.
- Subscriptions are either "explicit" (the user clicked…