Git merges in Moodle 3.6.4 by AMOS bot

commit 23d136271886eb0b7882c8d0f1d21ab9720d03ce
Author: AMOS bot <amos@moodle.org>
Date:   Wed, 10 Apr 2019 08:12:20 +0800
Tag:    v3.6.4

Merge branch 'MOODLE_36_STABLE' into install_36_STABLE