Fix emoji URLs in Markdown when relative_url_root is used
Path/URL helpers (e.g. `asset_url_helper`) already include the relative_url_root, so there is no need to attach it twice.
Also adds the ability to run rspecs with relative_url_defined on the enviornment. For example:
RELATIVE_URL_ROOT=/gitlab rspec
Closes#1728
See merge request !1254