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/activitypub/post_upgrade_worker.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class ActivityPub::PostUpgradeWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'pull'
+
+  def perform(domain)
+    Account.where(domain: domain)
+           .where(protocol: :ostatus)
+           .where.not(last_webfingered_at: nil)
+           .in_batches
+           .update_all(last_webfingered_at: nil)
+  end
+end