Add manage templates link to main navigation

Because it’s important

app/templates/main_nav.html

@@ -6,6 +6,7 @@
     <li><a href="{{ url_for('.sendsms', service_id=123) }}">Send text messages</a></li>
     <li><a href="{{ url_for('.sendemail', service_id=123) }}">Send emails</a></li>
     <li><a href="{{ url_for('.showjobs', service_id=123) }}">Activity</a></li>
+    <li><a href="{{ url_for('.manage_templates', service_id=123) }}">Manage templates</a></li>
   </ul>
   <ul>
     <li><a href="{{ url_for('.apikeys', service_id=123) }}">API keys and documentation</a></li>