Git commits in Moodle 3.6.4 by Bas Brands

commit bc270ce303bdcbf3b02b007f52df503900afb7c0
Author: Bas Brands <bas@moodle.com>
Date:   Wed, 24 Apr 2019 17:58:45 +0800
Tag:    v3.6.4

MDL-64267 core_message: fix z-index on sticky positioned notification


commit 57ec34a9ccb9b5de4265caaf11efdbb4c1e8b4df
Author: Bas Brands <bas@moodle.com>
Date:   Wed, 10 Apr 2019 16:43:48 +0800
Tag:    v3.6.4

MDL-64267 core_message: sticky positioning of contact request info


commit 828b40dca1d1967c355a6653144bedf0d1a7b04e
Author: Bas Brands <bas@moodle.com>
Date:   Wed, 12 Dec 2018 20:42:24 +0800
Tag:    v3.6.4

MDL-64102 core_message: add message tails to match ux design