Nuke message_bus_observer move to service class and classes

Secure all messages triggered by post creation and all user actions so they don't leak
(meaning, if you have a browser open and secure topics are created you will only get them if you are allowed to see them)

spec/models/user_action_spec.rb

@@ -91,11 +91,6 @@ describe UserAction do
     end
   end
 
-  it 'calls the message bus observer' do
-    MessageBusObserver.any_instance.expects(:after_create_user_action).with(instance_of(UserAction))
-    Fabricate(:user_action)
-  end
-
   describe 'when user likes' do
 
     let!(:post) { Fabricate(:post) }