Backport Gitlab::Ci::Model from EE

This backports the EE changes to Gitlab::Ci::Model back into CE,
ensuring the code is the same for both versions.

lib/gitlab/ci/model.rb

@@ -8,7 +8,7 @@ module Gitlab
       end
 
       def model_name
-        @model_name ||= ActiveModel::Name.new(self, nil, self.name.split("::").last)
+        @model_name ||= ActiveModel::Name.new(self, nil, self.name.demodulize)
       end
     end
   end