Update Mattermost to 5.9.0 (GitLab 11.10)
Mattermost 5.9 is a bug fix release, so it doesn't contain much in the way of new features, although it does include some low and medium level security fixes. The full changelog for it is here.
Merge requests to upgrade GitLab 11.9.x to Mattermost 5.8.1 and GitLab 11.8.x to Mattermost 5.7.3 will be coming shortly as well containing the fixes mentioned above.