Update Mattermost to 4.10.2 (GitLab 11.0)
Mattermost 4.10.2 contains a security fix that's also been backported to 5.0.2, so a followup PR will be coming for that. Let me know if I should be submitting this against a different branch or doing anything different with the changelog since it appears to have some GitLab 11.1 stuff in it as well
Edited by Harrison Healey