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/site_spec.rb

@@ -3,9 +3,6 @@ require_dependency 'site'
 
 describe Site do
   it "omits categories users can not write to from the category list" do
-
-    ActiveRecord::Base.observers.enable :anon_site_json_cache_observer
-
     category = Fabricate(:category)
     user = Fabricate(:user)