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