Use banners appropriately

We’ve fiddled around with the banners quite a lot in the last few days. This
commit reviews some of the older examples and makes sure that they’re:

a) not broken
b) using the most appropriate banner for the context

app/templates/views/styleguide.html

@@ -51,6 +51,8 @@
     type='tip'
   )}}
 
+  {{ banner('You could go to jail', 'important')}}
+
   <h2 class="heading-large">Big number</h2>
 
   <p>Used to show some important statistics.</p>