Update Mattermost to 5.8.2 (GitLab 11.9)
Mattermost 5.8.2 contains the security fixes mentioned in !3236 (merged) that have also been included in Mattermost 5.10 and Mattermost 5.9.1.
Mattermost 5.8.2 contains the security fixes mentioned in !3236 (merged) that have also been included in Mattermost 5.10 and Mattermost 5.9.1.