Add email placeholder positionid

src/pretix/base/email.py

@@ -646,6 +646,10 @@ def base_placeholders(sender, **kwargs):
             'attendee_name', ['position'], lambda position: position.attendee_name,
             _('John Doe'),
         ),
+        SimpleFunctionalMailTextPlaceholder(
+            'positionid', ['position'], lambda position: str(position.positionid),
+            '1'
+        ),
         SimpleFunctionalMailTextPlaceholder(
             'name', ['position_or_address'],
             get_best_name,