Updating GitLab Mattermost to 3.6 for GitLab 8.16 release
Replaces https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1221 and includes mattermost startup fix
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Replaces https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1221 and includes mattermost startup fix