Use atomUri in Undo activity of Announce (#5376)

This allows deletion of reblogs which delivered before with OStatus URI.

app/serializers/activitypub/activity_serializer.rb

@@ -5,6 +5,7 @@ class ActivityPub::ActivitySerializer < ActiveModel::Serializer
 
   has_one :proper, key: :object, serializer: ActivityPub::NoteSerializer, unless: :announce?
   attribute :proper_uri, key: :object, if: :announce?
+  attribute :atom_uri, if: :announce?
 
   def id
     ActivityPub::TagManager.instance.activity_uri_for(object)
@@ -34,6 +35,10 @@ class ActivityPub::ActivitySerializer < ActiveModel::Serializer
     ActivityPub::TagManager.instance.uri_for(object.proper)
   end
 
+  def atom_uri
+    OStatus::TagManager.instance.uri_for(object)
+  end
+
   def announce?
     object.reblog?
   end