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/notification_spec.rb

@@ -2,7 +2,6 @@ require 'rails_helper'
 
 describe Notification do
   before do
-    ActiveRecord::Base.observers.enable :all
     NotificationEmailer.enable
   end
 
@@ -235,7 +234,6 @@ describe Notification do
   describe 'ensure consistency' do
     it 'deletes notifications if post is missing or deleted' do
 
-      ActiveRecord::Base.observers.disable :all
       NotificationEmailer.disable
 
       p = Fabricate(:post)