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/post_timing_spec.rb

@@ -81,8 +81,7 @@ describe PostTiming do
     # integration test
 
     it 'processes timings correctly' do
-
-      ActiveRecord::Base.observers.enable :all
+      PostActionNotifier.enable
 
       post = Fabricate(:post)
       user2 = Fabricate(:coding_horror, created_at: 1.day.ago)