Add restricted mode warning

For the hack day, we should only let developers use the platform in restricted
mode. This commit adds a banner telling them this.

Can’t get the app running locally, so fingers crossed it actually looks how
I imagine it’s going to look…

app/templates/components/banner.html

@@ -1,5 +1,5 @@
-{% macro banner(body, with_tick=False) %}
-  <div class='banner{% if with_tick %}-with-tick{% endif %}'>
+{% macro banner(body, type=None, with_tick=False) %}
+  <div class='banner{% if type %}-{{ type }}{% endif %}{% if with_tick %}-with-tick{% endif %}'>
     {{ body }}
   </div>
 {% endmacro %}