Move Differential to proper subscriptions

Summary:
Ref T2222. Ref T4415. We're still writing Differential subscription stuff into this weird legacy `differential_relationship` table, which is like an edge table but extremely ancient.

Move it into a proper table.

I've removed `withSubscriptions()` from `DifferentialRevisionQuery`. It was weird, doesn't work consistently with other similar filters, and was only used by the API. Now it means "ccs", which is consistent with the ApplicationSearch UI and with Maniphest.

Test Plan:
Without migrating, added and removed subscribers via various workflows. Queried for subscribers. Everything worked as expected.

Ran the migration, verified data survived.

Reviewers: btrahan

Reviewed By: btrahan

CC: FacebookPOC, aran

Maniphest Tasks: T2222, T4415

Differential Revision: https://secure.phabricator.com/D8202