Updated Mattermost to 4.6.3 (GitLab 10.5)
This contains the security fixes for Mattermost 4.6.x mentioned in !2434 (merged). The earlier 4.6.1 and 4.6.2 releases didn't have any fixes that affected Mattermost Team Edition. Let me know if I should be submitting this against a different branch or doing anything with the changelog for this since the changelog in the branch appears to be out of date.