Include gitlab-org/gitlab-ui as part of gdk install/update

What does this Merge Request do?

This MR includes https://gitlab.com/gitlab-org/gitlab-ui when running gdk install or gdk update 🙂

To enable the gitlab-ui project to be cloned / updated as part of a gdk update, simply add the following to your gdk.yml

gitlab_ui:
  enabled: true

and run gdk update 🙂

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.

Closes #940 (closed)