Resolve "two visits with the same dates"

Closes #369 (closed), #373 (closed), #372 (closed), #370 (closed)

smash/web/templates/appointment_types/edit.html

-{% extends "languages/add_edit.html" %}
+{% extends "appointment_types/add_edit.html" %}
 
 {% block page_header %}Edit appointment type "{{ appointment_type.code }}"{% endblock page_header %}