Use standard rendering and controller for Differential subscriptions

Summary:
Ref T2222. Differential has custom code for managing subscriptions, but no longer requires it.

The one trick is that we don't have a hook for loading related data on the subscriptions workflow right now. Just glue that in for the moment; it's relatively harmless, and once Diffusion converts we'll have more context on how to best surface it properly.

Test Plan: Subscribed and unsubscribed from a revision. Viewed different revisions and saw correct subscription state.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8293