Add ActivityPub serializer for Undo of Announce (#4703)

app/serializers/activitypub/undo_announce_serializer.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class ActivityPub::UndoAnnounceSerializer < ActiveModel::Serializer
+  attributes :id, :type, :actor
+
+  has_one :object, serializer: ActivityPub::ActivitySerializer
+
+  def id
+    [ActivityPub::TagManager.instance.uri_for(object.account), '#announces/', object.id, '/undo'].join
+  end
+
+  def type
+    'Undo'
+  end
+
+  def actor
+    ActivityPub::TagManager.instance.uri_for(object.account)
+  end
+end