Strike-through lesson if cancelled. Closes #28.

biscuit/apps/alsijil/static/css/alsijil/lesson.css

@@ -3,3 +3,8 @@ input.alsijil-lesson-personal-note-name:disabled {
     border: 0;
     color: #212529;
 }
+
+.alsijil-lesson-cancelled {
+    background-color: inherit !important;
+    text-decoration: line-through;
+}

biscuit/apps/alsijil/templates/alsijil/group_week.html

@@ -47,6 +47,9 @@
                         {% else %}
                           {% if period.get_substitution %}
                             table-warning
+                            {% if period.get_substitution.cancelled %}
+                              alsijil-lesson-cancelled
+                            {% endif %}
                           {% endif %}
                         {% endif %}
                       {% endif %}