Rework some basics again

web/app/templates/materialize/base.html.j2

@@ -18,12 +18,22 @@
   </head>
   <body{% block body_attribs %}{% endblock body_attribs %}>
     {% block body %}
-    {% block navbar %}
-    {% endblock navbar %}
-    {% block sidenav %}
-    {% endblock sidenav %}
-    {% block content %}
-    {% endblock content %}
+    <header{% block header_attribs %}{% endblock header_attribs %}>
+      {% block header %}
+      {% block navbar %}
+      {% endblock navbar %}
+      {% block sidenav %}
+      {% endblock sidenav %}
+      {% endblock header %}
+    </header>
+
+    <main{% block main_attribs %}{% endblock main_attribs %}>
+      {% block main %}{% endblock main %}
+    </main>
+
+    <footer{% block footer_attribs %}{% endblock footer_attribs %}>
+      {% block footer %}{% endblock footer %}
+    </footer>
 
     {% block scripts %}
     <script src="{{ url_for('static', filename='js/materialize.min.js') }}"></script>