remove rails-observers

Rails yanked out observers many many years ago, instead the functionality
was yanked out to a gem that is very lightly maintained.

For example: if we want to upgrade to rails 5 there is no published gem

Internally the usage of observers had quite a few problem.

The series of refactors renamed a bunch of classes to give us more clarity
and removed some magic.

spec/models/user_action_spec.rb

@@ -3,7 +3,6 @@ require 'rails_helper'
 describe UserAction do
 
   before do
-    ActiveRecord::Base.observers.enable :all
     UserActionCreator.enable
   end
 
@@ -51,6 +50,8 @@ describe UserAction do
     end
 
     it 'includes the events correctly' do
+      PostActionNotifier.enable
+
       mystats = stats_for_user(user)
       expecting = [UserAction::NEW_TOPIC, UserAction::NEW_PRIVATE_MESSAGE, UserAction::GOT_PRIVATE_MESSAGE, UserAction::BOOKMARK].sort
       expect(mystats).to eq(expecting)