Refactor globalize models code using a concern

I've chosen the name "Globalizable" because "Translatable" already
existed.

app/models/concerns/globalizable.rb

@@ -0,0 +1,8 @@
+module Globalizable
+  extend ActiveSupport::Concern
+
+  included do
+    globalize_accessors
+    accepts_nested_attributes_for :translations, allow_destroy: true
+  end
+end