Git commits in Moodle 3.5.3 by Zig Tan

commit 6d03e5e5540a5196db48e1e4350de45f4e9bc619
Author: Zig Tan <zig@moodle.com>
Date:   Fri, 29 Jun 2018 12:25:41 +0800
Tag:    v3.5.3

MDL-61052 assign: Improve strip_images() to avoid HTML5 tags errors