This bumps gitlab to its latest version (12.1.6), also containing some important security fixes.
On top of that, we stop shipping the 1.3GiB gitlab-deb-data.tar file in $out ;-)

Motivation for this change

https://about.gitlab.com/2019/08/12/critical-security-release-gitlab-12-dot-1-dot-6-released/

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @lheckemann @talyz