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/controllers/admin/accounts_controller.rb

@@ -17,7 +17,7 @@ module Admin
     end
 
     def unsubscribe
-      UnsubscribeService.new.call(@account)
+      Pubsubhubbub::UnsubscribeWorker.perform_async(@account.id)
       redirect_to admin_account_path(@account.id)
     end