Updated Mattermost to 4.9.1
Mattermost 4.9.1 is a bug fix containing fixes for a couple performance regressions and for some issues with the new permissions system. The full change log is available here: https://docs.mattermost.com/administration/changelog.html#release-v4-9
Preferably, this would go into GitLab 10.8, but it's not critical if we've missed the release for that.