templates: Provide subsubmenu functionality in app.html

- So that eventually we can use just app.html instead of two: app.html and
  app-subsubmenu.html

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/templates/app.html

@@ -22,6 +22,7 @@
 
 {% load bootstrap %}
 {% load i18n %}
+{% load plinth_extras %}
 {% load static %}
 
 {% block content %}
@@ -46,6 +47,12 @@
 
   {% include "clients.html" with clients=clients enabled=is_enabled %}
 
+  {% block subsubmenu %}
+    {% if subsubmenu %}
+      {% show_subsubmenu subsubmenu %}
+    {% endif %}
+  {% endblock %}
+
   {% block status %}
     {% if show_status_block %}
       <h3>{% trans "Status" %}</h3>