Fix URL construction for merge requests, issues, notes, and commits for relative URL config

This MR fixes the constructor of URL links for systems using relative URLs in the application config. The `host` parameter was used improperly, supplying an extra path (e.g. https://mydomain.com/gitlab) and unnecessary scheme. We should be able to default to the URL options set in `8_default_url_options.rb` file.

Closes https://github.com/gitlabhq/gitlabhq/issues/9605

See merge request !1250