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/models/user_action_spec.rb

@@ -4,6 +4,7 @@ describe UserAction do
 
   before do
     ActiveRecord::Base.observers.enable :all
+    UserActionCreator.enable
   end
 
   it { is_expected.to validate_presence_of :action_type }