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

Send a notification email to user when
order is cancelled.

src/pretix/base/settings.py

@@ -316,6 +316,18 @@ Please note that this link is only valid within the next {hours} hours!
 We will reassign the ticket to the next person on the list if you do not
 redeem the voucher within that timeframe.
 
+Best regards,
+Your {event} team"""))
+    },
+    'mail_text_order_canceled': {
+        'type': LazyI18nString,
+        'default': LazyI18nString.from_gettext(ugettext_noop("""Hello,
+
+your order {code} for {event} has been canceled.
+
+You can view the details of your order at
+{url}
+
 Best regards,
 Your {event} team"""))
     },