Link wiki breadcrumbs to index pages

Eventually it might be a good idea to have these link to lists of pages
inside each folder (or maybe fall back to that if an index page doesn't
exist), but this is good for now.

tildes/tildes/templates/group_wiki_page.jinja2

@@ -20,7 +20,7 @@
   </li>
   {% for folder in page.folders %}
     <li class="breadcrumb-item">
-      <a href="{{ request.route_url("group_wiki_page", group_path=page.group.path, wiki_page_path=folder|string) }}">{{ folder.name }}</a>
+      <a href="{{ request.route_url("group_wiki_page", group_path=page.group.path, wiki_page_path=folder~"/index") }}">{{ folder.name }}</a>
     </li>
   {% endfor %}
 </ol>