ActivityPub migration procedure (#4617)

* ActivityPub migration procedure

Once one account is detected as going from OStatus to ActivityPub,
invalidate WebFinger cache for other accounts from the same domain

* Unsubscribe from PuSH updates once we receive an ActivityPub payload

* Re-subscribe to PuSH unless already unsubscribed, regardless of protocol

app/workers/scheduler/subscriptions_scheduler.rb

@@ -14,6 +14,6 @@ class Scheduler::SubscriptionsScheduler
   private
 
   def expiring_accounts
-    Account.where(protocol: :ostatus).expiring(1.day.from_now).partitioned
+    Account.expiring(1.day.from_now).partitioned
   end
 end