Fix #543 -- Allow send mail from order (#550)

- Add send email directly for order
- Add email history (from mass and custom) to each specific order

src/pretix/base/settings.py

@@ -346,6 +346,16 @@ your order {code} for {event} has been canceled.
 You can view the details of your order at
 {url}
 
+Best regards,
+Your {event} team"""))
+    },
+    'mail_text_order_custom_mail': {
+        'type': LazyI18nString,
+        'default': LazyI18nString.from_gettext(ugettext_noop("""Hello,
+
+You can change your order details and view the status of your order at
+{url}
+
 Best regards,
 Your {event} team"""))
     },