When creating GitLab group, create Mattermost team

Assume your Mattermost instance is already talking to your GitLab instance.
Add a Create a Mattermost team for this group option to the New group page, which allows users to create a Mattermost team when creating a GitLab group
If Mattermost is not configured, this option is not available.
If Mattermost is configured, this option is available and selected by default.
The Mattermost team is based on the group name and URL of the group (e.g. 'gitlab-org' group creates a 'gitlab-org' Mattermost team)
The entire process is part of the group creation transaction. If there is any failure, the whole thing fails and no group is persisted.
Because this process can be instant or take a while, a loading screen is shown after pressing the Create group button
If the Mattermost creation part fails due to some error (e.g. naming restrictions):
- the user is brought back to the New group page (the form is intact, no info is lost),
- with an error message, containing a link to the Mattermost instance team creation page: http://<mattermost-url>/create_team
- and the Mattermost team creation option is unselected

This is just an automated step as part of GitLab group creation. The Mattermost team is not deeply linked to the group. So if you go back to the group page to edit it later, there is no Mattermost team associated with it in the UI.
If you are editing a group, you do not get any of these features. This is just during group creation.

New group

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