Don't allow a relative_url_root of '/'
Don't allow a relative_url_root of '/'

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.

stoplightio/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,iiet/iiet-git