manager: add an informational message when site is empty

combo/manager/templates/combo/manager_home.html

@@ -7,6 +7,7 @@
 
 {% block content %}
 
+{% if object_list %}
 <div id="pages-list" data-page-order-url="{% url 'combo-manager-page-order' %}">
  {% for page in object_list %}
  <div class="level-{{page.level}}" data-page-id="{{page.id}}" data-level="{{page.level}}">
@@ -14,5 +15,11 @@
  </div>
  {% endfor %}
 </div>
+{% else %}
+<div class="big-msg-info">
+  This page doesn't have any page yet. Click on the "New" button in the top
+  right of the page to add a first one.
+</div>
+{% endif %}
 
 {% endblock %}