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.

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

Ran the migration, verified data survived.