Use yellow warning banner for permissions message

The green bordered banner feels too much like ‘success’ or
‘confirmation’. Doesn’t feel like it’s something which just gives you
the status of a thing, or here’s a thing you should be aware of.

app/assets/stylesheets/components/banner.scss

@@ -160,7 +160,7 @@
   background: $yellow;
   color: $text-colour;
   border: 5px solid $text-colour;
-  margin: $gutter 0 $gutter 0;
+  margin: $gutter-half 0 $gutter 0;
   text-align: left;
   padding: 20px;