Add page where users can say they want branding

At the moment branding is an undocumented feature. We get a bunch of
support tickets from teams asking its possible.

This commit:
- lets people know it’s possible, and what the options are
- is the first step towards making this process as self-service as
  possible

In some cases we will be able to infer a user’s organisation from there
email address, and Google image search their logo. So the experience for
them is that they press a button and government just sorts it out for
you (also known as "the dream").

In other cases we will have to get back to people asking for a copy of
their logo, or to find out about their service, but this is what we have
to do at the moment anyway.

app/assets/stylesheets/app.scss

@@ -241,3 +241,13 @@ details .arrow {
   }
 
 }
+
+.bordered-image {
+  padding: 5px;
+  outline: 1px solid $border-colour;
+  max-width: 100%;
+
+  label & {
+    cursor: pointer;
+  }
+}