Show a message when there are no visualisations

If there are no visualisations to show on the visualisations page, show
 a message rather than just an empty page to avoid ambiguity for the
 user.

Author: mtaylorgds

templates/visualisations.html

@@ -11,13 +11,17 @@
       <h1>Visualisations</h1>
     </header>
     <main>
+      {% if visualisations %}
       <ul class="visualisation-list">
         {% for v in visualisations %}
         <li>
           <a href="{{ url_for('graph_page', source_path=v.source_path) }}">{{ v.source_path }}</a>
         </li>
         {% endfor %}
       </ul>
+      {% else %}
+      <p>There are no visualisations to show</p>
+      {% endif %}
     </main>
   </body>
 </html>