Run on a relative URL root by default

GitLab supports being run on a relative URL, but we frequently introduce changes that don't take account of this functionality:

• https://gitlab.com/gitlab-org/gitlab-ee/issues/4658
• https://gitlab.com/gitlab-org/gitlab-ce/issues/45649

Let's introduce a relative_url_root file into GDK, defaulting to /gitlab, so that developers will find their code breaking if they don't take account of it.