Git commits in Moodle 3.8.1 by John Yao

commit b423c5ad383edc39314bab2b0a2229461a36aa8c
Author: John Yao <johnyao@catalyst-au.net>
Date:   Mon, 16 Dec 2019 08:28:04 +0800
Tag:    v3.8.1

MDL-67524 book: fixing view hidden chapter issue