Hide ‘back to …’ link if it’s not your service

This can happen if you click a link for a service you don’t have access
to. We shouldn’t show the back to service link in this case because:
- you shouldn’t be able to find out the service’s name from just knowing
  the link
- if you click the link you only get a `403` anyway

app/templates/withoutnav_template.html

@@ -2,7 +2,7 @@
 
 {% block fullwidth_content %}
   <div id="content">
-    {% if current_service and current_user.is_authenticated %}
+    {% if current_service and current_user.is_authenticated and current_user.belongs_to_service(current_service.id) %}
     <div class="navigation-service">
       <a href="{{ url_for('main.show_accounts_or_dashboard') }}">Back to {{ current_service.name }}</a>
     </div>