Add function to cancel lessons. Closes #25.

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

@@ -4,7 +4,7 @@
   {% if lesson_period.get_substitution.cancelled %}
     border border-danger chronos-lesson-cancelled
   {% elif lesson_period.get_substitution %}
-   border border-warning
+    border border-warning
   {% endif %}
   "
   style="