Add setting uniqueness test

config/locales/en.yml

@@ -64,6 +64,10 @@ en:
               invalid: 'IPv4 is invalid'
             ipv6:
               invalid: 'IPv6 is invalid'
+        setting:
+          attributes:
+            code:
+              taken: 'Code already exists'
     attributes:
       domain:
         name: 'Domain name'

config/routes.rb

@@ -6,7 +6,6 @@ Rails.application.routes.draw do
   end
 
   resources :setting_groups
-  resources :settings
 
   # The priority is based upon order of creation: first created -> highest priority.
   # See how all your routes lay out with "rake routes".