Update Mattermost to 5.0.0
Mattermost 5.0 has been tested with GitLab SSO with official release on June 15 from https://about.mattermost.com/download/. I've tested it with a version of GitLab from yesterday. Full release notes for Mattermost 5.0 are here, although most of the breaking changes were accounted for in GitLab 11, such as the deprecation of API v3.