Use asset fingerprinter to serve images

This means we can cache them forever, and only invalidate the cache
when the image changes.

app/templates/views/service-settings/branding/email-options.html

@@ -26,7 +26,7 @@
         {% for option in form.options %}
           <div class="column-one-quarter">
             <label for="{{ option.id }}">
-              <img src="{{ asset_path }}images/branding-options/{{ option.data }}.png" alt="" class="bordered-image bottom-gutter-1-3">
+              <img src="{{ asset_url('images/branding-options/{}.png'.format(option.data)) }}" alt="" class="bordered-image bottom-gutter-1-3">
             </label>
             {{ radio(option) }}
           </div>