Git commits in Moodle 3.9.0 by Martin Gauk

commit 966556074ce9896009b17c157944c694425f3cd9
Author: Martin Gauk <gauk@math.tu-berlin.de>
Date:   Fri, 17 Jan 2020 20:13:41 +0800
Tag:    v3.9.0-beta

MDL-51694 core_grades: use transactions when deleting