Ship Mattermost EE with GitLab EE
These changes will make it so that GitLab EE will ship with Mattermost EE instead of Mattermost TE. GitLab CE will still ship with Mattermost TE as it did before. Installation and such is the same except it will swap out the server binary and the license file when building Omnibus since those are the only differences between the Mattermost TE and EE packages.
Since I don't think I can build GitLab EE myself, I've tested this locally by building GitLab CE and by building with the check for GitLab EE forced to true. I'd appreciate some help testing how it actually works with GitLab EE although I assume it won't be majorly different