Release 2.7.2 for Moodle 2.7 and 2.8

This is a bug fix release with no features added.

Main fixes: forced groupmode seeting (CONTRIB-5976), incorrect recipient for notification when booking several slots

This release is intended for Moodle 2.7 and 2.8. Users of Moodle 2.9 and later should upgrade to Scheduler 2.9.