Update to Mattermost 4.5.1 (GitLab 10.4)
Mattermost 4.5.1 was released on January 16 from https://about.mattermost.com/download/. We've tested it with the latest version of GitLab from the 10.4 branch.
This is just a bug fix release to fix an issue where Mattermost didn't work on certain versions of Firefox. See #3099 (closed) for more details.