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/pubsubhubbub/unsubscribe_worker.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class Pubsubhubbub::UnsubscribeWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'push', retry: false, unique: :until_executed, dead: false
+
+  def perform(account_id)
+    account = Account.find(account_id)
+    logger.debug "PuSH unsubscribing from #{account.acct}"
+    ::UnsubscribeService.new.call(account)
+  rescue ActiveRecord::RecordNotFound
+    true
+  end
+end