Fix css classes.

biscuit/apps/chronos/templates/chronos/tt_lesson.html

 {# -*- engine:django -*- #}
 
 <div class="card chronos-lesson
-  {% if lesson_period.get_substitution %}
-  border border-warning
-  {% endif %}
   {% if lesson_period.get_substitution.cancelled %}
   border border-danger chronos-lesson-cancelled
+  {% elif lesson_period.get_substitution %}
+   border border-warning
   {% endif %}
   "
   style="