Add support for more generic notification

This is a start to have different kinds of notifications. It is
possible to be notified via e-mail, matrix or gotify. It also extends
the current "periodic query" for due items by allowing notification
over different channels. A "generic periodic query" variant is added
as well.

website/site/templates/overview.html

@@ -36,6 +36,7 @@
                     {% for section in section.subsections %}
 
                     {% set sub = get_section(path=section) %}
+                    {% if not sub.extra.hidden %}
 
                     <div class="column is-one-third-widescreen is-half-tablet">
                         <div class="card full-height">
@@ -55,6 +56,8 @@
                             </a>
                         </div>
                     </div>
+                    {% endif %}
+
                     {% endfor %}
                 </div>
             </div>