Sync GitLab group to Mattermost team

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Description

We support creating a Mattermost team during GitLab group creation, but we don't keep that team's members updated with members of the GitLab group. We should.

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.)