use mptt's get_descendant_count for the "has children" calculatio…

…n, which doesn't make a query

cms/templates/admin/cms/page/lazy_child_menu.html

@@ -1,8 +1,8 @@
-{% load cms_admin i18n adminmedia %}{% with page.children.exists as has_children %}<li id="page_{{page.pk}}" class="{% if cl.is_filtered %}leaf{% endif %}{% if has_move_page_permission %} moveable{% endif %}{% if has_children %} closed{% endif %}"{% if metadata %} mdata="{{ metadata }}{% endif %}" rel="{% ifequal page.level 0 %}topnode{% else %}node{% endifequal %}">
+{% load cms_admin i18n adminmedia %}{% with page.get_descendant_count as has_children %}<li id="page_{{page.pk}}" class="{% if cl.is_filtered %}leaf{% endif %}{% if forloop.last %} last{% endif %}{% if has_move_page_permission %} moveable{% endif %}{% if has_children %} closed{% endif %}"{% if metadata %} mdata="{{ metadata }}{% endif %}" rel="{% ifequal page.level 0 %}topnode{% else %}node{% endifequal %}">
 
     {% include 'admin/cms/page/menu_item.html' %}
 
     {% if has_children %}<ul{% if page.last %} class="last"{% endif %}>
     </ul>{% endif %}
 </li>
-{%  endwith %}
+{%  endwith %}