Fix #391 -- Send email on cancelled order (#540)

Send a notification email to user when
order is cancelled.

src/pretix/control/forms/event.py

@@ -558,6 +558,13 @@ class MailSettingsForm(SettingsForm):
         help_text=_("Available placeholders: {event}, {url}, {product}, {hours}, {code}"),
         validators=[PlaceholderValidator(['{event}', '{url}', '{product}', '{hours}', '{code}'])]
     )
+    mail_text_order_canceled = I18nFormField(
+        label=_("Text"),
+        required=False,
+        widget=I18nTextarea,
+        help_text=_("Available placeholders: {event}, {code}, {url}"),
+        validators=[PlaceholderValidator(['{event}', '{code}', '{url}'])]
+    )
     smtp_use_custom = forms.BooleanField(
         label=_("Use custom SMTP server"),
         help_text=_("All mail related to your event will be sent over the smtp server specified by you."),

src/pretix/control/templates/pretixcontrol/event/mail.html

@@ -35,6 +35,10 @@
 
                 {% blocktrans asvar title_waiting_list_notification %}Waiting list notification{% endblocktrans %}
                 {% include "pretixcontrol/event/mail_settings_fragment.html" with pid="waiting_list" title=title_waiting_list_notification items="mail_text_waiting_list" %}
+
+                {% blocktrans asvar title_order_canceled %}Order canceled{% endblocktrans %}
+                {% include "pretixcontrol/event/mail_settings_fragment.html" with pid="order_canceled" title=title_order_canceled items="mail_text_order_canceled" %}
+
             </div>
         </fieldset>
         <fieldset>

src/pretix/control/views/event.py

@@ -439,7 +439,8 @@ class MailSettingsPreview(EventPermissionRequiredMixin, View):
             'mail_text_resend_all_links': ['event', 'orders'],
             'mail_text_order_changed': ['event', 'url', 'invoice_name', 'invoice_company'],
             'mail_text_order_expire_warning': ['event', 'url', 'expire_date', 'invoice_name', 'invoice_company'],
-            'mail_text_waiting_list': ['event', 'url', 'product', 'hours', 'code']
+            'mail_text_waiting_list': ['event', 'url', 'product', 'hours', 'code'],
+            'mail_text_order_canceled': ['code', 'event', 'url'],
         }
 
     @cached_property