Move service and user nav to proposition header

This commit moves user-related navigation into the proposition header (the black
bar) at the top of the site. It adds some custom SASS to override GOV.UK
template and align these navigation items to the right (because it looks
better).

It then removes the service chooser dropdown (and its associated SASS and JS) in
favour of a link alongside the user-related navigation items. ‘Switch service’
is the best language for this that we’ve come up with so far.

This means that the only way of adding a new service is from the `/services`
page. So this commit removes the redirect if you land on this page with only one
service (else it would prevent you from ever being able to add more).

app/templates/main_nav.html

@@ -1,4 +1,7 @@
 <nav class="navigation">
+  <h2 class="navigation-service-name">
+    {{ session.get('service_name', 'Choose service') }}
+  </h2>
   <ul>
     <li><a href="{{ url_for('.service_dashboard', service_id=service_id) }}">Dashboard</a></li>
   </ul>