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