Put checkered background behind email branding

This will stop images which are white with a transparent background
being invisible in the admin app (and accidentally getting overwritten).

app/templates/components/radios.html

@@ -98,7 +98,7 @@
         {% endif %}
       </legend>
       {% for value, option, checked in field.iter_choices() %}
-        <div class="multiple-choice">
+        <div class="multiple-choice branding-radio">
           <input
             type="radio"
             name="{{ field.name }}"