gitlab: Refactor for new repo structure

GitLab recently restructured their repos; whereas previously they had
one gitlab-ce and one gitlab-ee repo, they're now one and the
same. All proprietary components are put into the ee subdirectory -
removing it gives us the foss / community version of GitLab. For more
info, see
https://about.gitlab.com/2019/02/21/merging-ce-and-ee-codebases/

This gives us the opportunity to simplify things quite a bit, since we
don't have to keep track of two separate versions of either the base
data or rubyEnv.