core: simplify contest & homework logic

packages/ui-default/templates/contest_detail.html

@@ -30,6 +30,11 @@
           <p>{{ _('We will check code similarity after this contest.') }}</p>
         </blockquote>
       {% endif %}
+      {% if tdoc.duration %}
+        <blockquote class="note">
+          <p>{{ _('The contest is a flexible time contest. You need to complete the contest within a specified time after you attended.') }}</p>
+        </blockquote>
+      {% endif %}
       </div>
     </div>
   {% if model.contest.isOngoing(tdoc, tsdoc) or model.contest.isDone(tdoc, tsdoc) %}