deny creation of mattermost team for new group

Summary

As admins of a self hosted gitlab with tons of students, we decided that we only want admins to be able to generate mattermost teams. We deactivated team creation on mattermost side (earlier with the mattermost['team_enable_team_creation'] = false, now via the mattermost system console settings and additionally via mattermost['env'] = {..., 'MM_TEAMSETTINGS_ENABLETEAMCREATION' => 'false',). However, we recently noticed that some students had managed to create teams.

Looking into this, i discovered that creating new teams has a checkbox (sadly even auto-enabled gitlab-ce#33937) to "Create a Mattermost team for this group". Sadly, this allows students to circumvent all our attempts to hinder team creation. I think that gitlab here acts as a mattermost admin and hence is allowed to create the new team.

Steps to reproduce

Create a new team, click the checkbox.

What is the current bug behavior?

New mattermost team is created without admin's ability to hinder this.

What is the expected correct behavior?

Some setting (config or gitlab admin area) to disallow mattermost team creation from gitlab by normal users.

Relevant logs and/or screenshots

Results of GitLab environment info

Omnibus 10.8.3

Possible source of regression

Somewhere in code introduced for gitlab-ce#23964