Upgrading to GitLab Mattermost in GitLab 8.9

This is a thread started to help troubleshoot issues with upgrading to GitLab 8.9, which upgrades users to Mattermost 3.1.

If you're having issues with the automated install, try running:

sudo -u mattermost -i bash
cd /opt/gitlab/embedded/service/mattermost
/opt/gitlab/embedded/bin/mattermost -config='/var/opt/gitlab/mattermost/config.json' -upgrade_db_30

If this doesn't work, please post a new topic to this forum with:

1. A brief description of your issue as title
2. The step on which you're having issues
3. The relevant portion of the Mattermost log file at /var/log/gitlab/mattermost/mattermost.log