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/assets/stylesheets/admin/locales/form.scss

@@ -7,6 +7,10 @@
     width: auto;
   }
 
+  .help-text {
+    display: block;
+  }
+
   > select + fieldset,
   > fieldset + fieldset {
     margin-top: $line-height / 2;