Use Tombstone and _:atomUri in Delete activities as fallback (#4704)

app/serializers/activitypub/delete_serializer.rb

@@ -1,8 +1,26 @@
 # frozen_string_literal: true
 
 class ActivityPub::DeleteSerializer < ActiveModel::Serializer
+  class TombstoneSerializer < ActiveModel::Serializer
+    attributes :id, :type
+    attribute :atom_uri, key: '_:atomUri'
+
+    def id
+      ActivityPub::TagManager.instance.uri_for(object)
+    end
+
+    def type
+      'Tombstone'
+    end
+
+    def atom_uri
+      ::TagManager.instance.uri_for(object)
+    end
+  end
+
   attributes :id, :type, :actor
-  attribute :virtual_object, key: :object
+
+  has_one :object, serializer: TombstoneSerializer
 
   def id
     [ActivityPub::TagManager.instance.uri_for(object), '#delete'].join
@@ -15,8 +33,4 @@ class ActivityPub::DeleteSerializer < ActiveModel::Serializer
   def actor
     ActivityPub::TagManager.instance.uri_for(object.account)
   end
-
-  def virtual_object
-    ActivityPub::TagManager.instance.uri_for(object)
-  end
 end