Add missing ActivityPub representations (#4230)

- Follow, undo follow
- Accept follow, reject follow
- Like, undo like
- Block, undo block
- Delete (note)
- Update (actor)

app/serializers/activitypub/undo_like_serializer.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class ActivityPub::UndoLikeSerializer < ActiveModel::Serializer
+  attributes :type, :actor
+
+  has_one :object, serializer: ActivityPub::LikeSerializer
+
+  def type
+    'Undo'
+  end
+
+  def actor
+    ActivityPub::TagManager.instance.uri_for(object.account)
+  end
+end