Hide maintenance-mode content from ToC

Fixes #1124
Fixes #438

Author: ihalaij1

exercise/templates/exercise/_children.html

@@ -9,13 +9,13 @@
 {% elif entry.up %}
 </ul>
 {% endif %}
-{% elif entry|is_listed %}
+{% elif entry|is_listed and not entry|is_in_maintenance %}
 <li>
   {% if accessible and not entry.submittable or exercise_accessible %}
   {% if entry.is_empty %}
     {{ entry.name|parse_localization }}
   {% else %}
-    <a href="{{ entry.link }}" class="{% if entry|is_in_maintenance %}maintenance{% endif %}">{{ entry.name|parse_localization }}</a>
+    <a href="{{ entry.link }}">{{ entry.name|parse_localization }}</a>
   {% endif %}
   {% if entry.submission_count %}
     {% points_badge entry %}