Clean install of Gitlab 8.9 (not upgrade), enable Mattermost, Bad Gateway

New in 8.9.3, you must as root

cd /opt/gitlab/embedded/service/mattermost
sudo -u mattermost -i bash

And now continuing as mattermost, change back to the same mattermost folder:

cd /opt/gitlab/embedded/service/mattermost

Now see if the "platform" tool (what a stupid ass name for an application) which was renamed to the much more sensible "mattermost" name in the context of the mattermost-bundled-in-gitlab) actually runs by running this and seeing if it panics, like Go code seems to love to do at the least excuse:

$ /opt/gitlab/embedded/bin/mattermost -config=/var/opt/gitlab/mattermost/config.json -version

If you get the version output then the piece of Go code is not feeling shy and may then be usable to do other stuff like:

 /opt/gitlab/embedded/bin/mattermost -config=/var/opt/gitlab/mattermost/config.json -upgrade_db_30

However that's still not working. The database is both upgraded and not upgraded. We have tea and no tea. We are caught in between worlds.

The mattermost logs says we can't run because the database is at schema level 2.2.
The upgrade tool says the database appears to be upgraded.

AAAAUGH.