Add federation relay support (#7998)

* Add federation relay support

* Add admin UI for managing relays

* Include actor on relay-related activities

* Fix i18n

app/serializers/activitypub/delete_serializer.rb

@@ -17,7 +17,7 @@ class ActivityPub::DeleteSerializer < ActiveModel::Serializer
     end
   end
 
-  attributes :id, :type, :actor
+  attributes :id, :type, :actor, :to
 
   has_one :object, serializer: TombstoneSerializer
 
@@ -32,4 +32,8 @@ class ActivityPub::DeleteSerializer < ActiveModel::Serializer
   def actor
     ActivityPub::TagManager.instance.uri_for(object.account)
   end
+
+  def to
+    [ActivityPub::TagManager::COLLECTIONS[:public]]
+  end
 end