Hide maintenance-mode content from ToC

Fixes #1124
Fixes #438

Author: ihalaij1

exercise/cache/hierarchy.py

@@ -302,7 +302,10 @@ def is_listed(cls, entry):
             return (
                 entry.get('category_status') != LearningObjectCategory.STATUS.HIDDEN
                 and entry.get('module_status') != CourseModule.STATUS.UNLISTED
-                and entry['status'] != LearningObject.STATUS.UNLISTED
+                and not entry['status'] in (
+                    LearningObject.STATUS.UNLISTED,
+                    LearningObject.STATUS.MAINTENANCE,
+                )
             )
         if t == 'module':
             return entry['status'] != CourseModule.STATUS.UNLISTED

exercise/templates/exercise/_children.html

@@ -15,7 +15,7 @@
   {% 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 %}