Extract model to handle locales settings

This way we can simplify the view, particularly the form. However, we're
still adding some complexity to the form so inputs are inside labels and
so the collection is easier to style with CSS.

app/controllers/admin/locales_controller.rb

@@ -1,15 +1,27 @@
 class Admin::LocalesController < Admin::BaseController
+  before_action :set_locales_settings
+  authorize_resource :locales_settings
+
   def show
-    @enabled_locales = Setting.enabled_locales
-    @default_locale = Setting.default_locale
   end
 
   def update
-    Setting.transaction do
-      Setting["locales.default"] = params["default_locale"]
-      Setting["locales.enabled"] = [params["default_locale"], *params["enabled_locales"]].join(" ")
-    end
+    @locales_settings.update!(locales_settings_params)
 
     redirect_to admin_locales_path, notice: t("admin.locales.update.notice")
   end
+
+  private
+
+    def locales_settings_params
+      params.require(:setting_locales_settings).permit(allowed_params)
+    end
+
+    def allowed_params
+      [:default, enabled: []]
+    end
+
+    def set_locales_settings
+      @locales_settings = Setting::LocalesSettings.new
+    end
 end