Use unversioned slide link from the materials popup from the agenda page. Fixes ietf-tools#2756. Commit ready for merge.

 - Legacy-Id: 16594

Author: rjsparks

ietf/templates/meeting/session_agenda_include.html

@@ -33,7 +33,7 @@ <h4>Slides</h4>
                       {% for slide in item.session.slides %}
                         <li>
                           <span class="fa-li fa fa-file-{{slide.file_extension|lower}}-o"></span>
-                          <a href="{{ slide.href }}">{{ slide.title|clean_whitespace }}</a>
+                          <a href="{{ slide.gref }}">{{ slide.title|clean_whitespace }}</a>
                         </li>
                       {% endfor %}
                     </ul>