templates: Allow building pages without navigation bar and footer

Tests:

- Functional tests of many apps pass with the patch.

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

plinth/templates/base.html

@@ -84,6 +84,7 @@
       {% if refresh_page_sec is not None %}
         data-refresh-page-sec="{{ refresh_page_sec }}"
       {% endif %}>
+{% block wrapper %}
 <div id="wrapper">
   <div class="main-header fixed-top">
     <div class="container">
@@ -283,5 +284,6 @@
     </div>
   </footer><!--/.footer-->
 </div><!--/#container-->
+{% endblock %}
 </body>
 </html>