Only show ‘back to service name’ link if signed in

When you’re not signed in you can still have a service in your session.
But there’s no point trying to get to its dashboard until you’re signed
in – you’ll just be sent back to the ‘sign in’ page.

app/templates/withoutnav_template.html

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