Include mmctl in /opt/gitlab/embedded/bin

Summary

Include mmctl in /opt/gitlab/embedded/bin.

Proposal

As of Mattermost 6.0, the old mattermost command line tool has had most of its functionality removed in favor of the new mmctl command line tool.

Since this upgrade, it is no longer possible to delete mattermost users (among other things) without tampering with the mattermost database by hand.

References

1. 1 Deprecation warning for most bin/mattermost features (in the (!) Note box)
2. 2 Documentation for mmctl including a list of commands it includes.