Add session description to the course page

See also pyvec#372

Author: hroncok

naucse/templates/content/course.html

@@ -24,6 +24,9 @@ <h4>
             <small>({{ session.date | format_date }})</small>
         {% endif %}
     </h4>
+        {% if session.description %}
+            {{ session.description | markdown }}
+        {% endif %}
     {{ material_list(session.materials) }}
 </div>
 {% endfor %}