Git commits in Moodle 3.6.0 by Simon Champion

commit 99405aa7e2a34174a3eeaf9f9ffc9db3bc9f6192
Author: Simon Champion <simonc@sfwltd.co.uk>
Date:   Thu, 29 Mar 2018 19:00:08 +0800
Tag:    v3.6.0-beta

MDL-61694 core: Add callback to allow additional password check.