Show groups intended by depth in pulldown menu

Use Group.get_depth_fill method to get some indention into the group
pulldown menu.

mpicms/templates/personal/list.html

@@ -21,7 +21,7 @@ <h1 class="title has-text-centered">{% trans 'Contact List' %}</h1>
                 <div class="dropdown-content">
                 {% for group in groups %}
                     <a href="?group={{ group.pk }}" class="{% if group.pk|stringformat:'d' == selected_group_pk %}is-active {% endif %}dropdown-item">
-                        {{ group }}
+                        {{ group.get_depth_fill }} {{ group }}
                     </a>
                 {% endfor %}
                 </div>