Update Mattermost to 5.6.4 (GitLab 11.7)
Mattermost 5.6.4 contains the security fixes mentioned in !3019 (merged) that are also included with 5.7.1 and 5.5.3.
It's been a while since I've submitted any backports, so let me know if I should change how I'm doing this since I'll also be submitting a similar MR for upgrading GitLab 11.6 to use Mattermost 5.5.3
Edited by 🤖 GitLab Bot 🤖