Resolve "Referencing issues or epics by URL fails with 404"

What does this MR do?

This commit prevents the relative_url_root being '/'. This will fail in a few ways:

1. We might end up having a path (not a URL) starting with //, which will be interpreted by browsers as a protocol-relative URL.
2. Issue, MR, snippet, etc. reference parsing will look for URLs at http://gitlab.example.com//project/..., with the double slash preventing single slashes from working.

In general, it doesn't seem like there's a valid case for this.

Closes #60965 (closed) and https://gitlab.com/gitlab-org/gitlab-ce/issues/61099.