Delete Mattermost team when deleting GitLab group

Description

If we automatically created a Mattermost team during GitLab group creation, we should automatically destroy it when the group is destroyed. This may not happen often in practice, but comes up when doing our sales demo which creates lots of groups.

Proposal

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)