gitlab: 12.1.6 -> 12.3.4

- Update GitLab to 12.3.4

- Update update.py to cope with the new upstream repository structure

- Refactor gitlab-shell to use buildGoPackage and bundlerEnv for
  dependencies

- Refactor gitlab-workhorse to use buildGoPackage for dependencies

- Make update.py able to update gitlab-shell and gitlab-workhorse
  dependencies

- Various fixes necessary for update to work