Show if lesson is cancelled. Closes #31.

biscuit/apps/chronos/tables.py

@@ -14,6 +14,10 @@ def _css_class_from_lesson_state(record: Optional[LessonPeriod] = None, table: O
         return 'table-warning'
     else:
         return ''
+    if record.get_substitution(record._week).cancelled:
+        return 'table-danger'
+    else:
+        return ''
 
 
 class LessonsTable(tables.Table):