Add gitlab and gitlab-shell.

I had to make several adjustments to make it work with nixos:

* Replace relative config file lookups with ENV variable.
* Modify gitlab-shell to not clear then environment when running
  pre-receive.
* Modify gitlab-shell to write some environment variables into
  the .authorized_keys file to make sure gitlab-shell reads the
  correct config file.

The current version requires the user to install the gitlab-shell
package into the systemPackages environment because
/run/current-system/sw/bin/gitlab-shell is hard-coded in the
.authorized_keys file.
It's been a few weeks so my memory is already hazy but I think I couldn't get bundler to read the whole rails config directory from anywhere else other than the execution point (i.e. in a read-only nix-store path).

I'd be super happy if someone knowledgeable would make this cleaner!

I packaged sentry for our website and it's just so much nicer because it takes single config file on execution. None of the relative importing that rails seems to do: https://github.com/WeAreWizards/website/tree/master/sentry