FIX: Couldn't migrate database from nothing due to I18n

Since I18n has a DB backend now, I've introduced a helper we can use to
skip overrides in certain situations. Otherwise migration from empty
databases was broken.

db/migrate/20140227201005_add_staff_category.rb

@@ -1,6 +1,8 @@
 class AddStaffCategory < ActiveRecord::Migration
   def up
-    unless Rails.env.test?
+    return if Rails.env.test?
+
+    I18n.backend.overrides_disabled do
       result = Category.exec_sql "SELECT 1 FROM site_settings where name = 'staff_category_id'"
       if result.count == 0
         description = I18n.t('staff_category_description')