Add function to cancel lessons. Closes #25.

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

@@ -2,7 +2,7 @@
 
 <div class="card chronos-lesson
   {% if lesson_period.get_substitution.cancelled %}
-  border border-danger chronos-lesson-cancelled
+    border border-danger chronos-lesson-cancelled
   {% elif lesson_period.get_substitution %}
    border border-warning
   {% endif %}