Updated GitLab Mattermost docs for GitLab 11
I've updated the GitLab Mattermost documentation for GitLab 11 to point users towards using the System Console in Mattermost or the config.json to configure it. I left in the section about configuring Mattermost through the environment variables since the warning in GitLab has been telling people to do that for a while, but I moved it after the instructions to just use the System Console.
Ticket Link
#3284 (closed)
https://mattermost.atlassian.net/browse/MM-10014
Edited by GitLab Release Tools Bot