remove UserActionObserver, replace with after_save and service

interestingly there was some left over dead code from when stars
existed in the topic_users table

spec/controllers/user_actions_controller_spec.rb

@@ -10,6 +10,7 @@ describe UserActionsController do
 
     it 'renders list correctly' do
       ActiveRecord::Base.observers.enable :all
+      UserActionCreator.enable
       post = Fabricate(:post)
 
       xhr :get, :index, username: post.user.username