Addded the session description to the coverpage

Cherry-pick of commit 1f45855
(#372)
Updated for model with sanitized HTML

Fixes: #434

Author: mikicz

naucse/templates/coverpage.html

@@ -21,6 +21,8 @@ <h1>
         {{ session.time.start | format_time }}–{{ session.time.end | format_time }}</div>
 {% endif %}
 
+{% if session.description %}{{ session.description }}{% endif %}
+
 <h2>Materiály</h2>
 {{ material_list(session.materials, filter_type='lesson') }}