getDomainFromUrl(url) {
        const hostname = url.hostname;
        const domain = hostname.replace(/^www\./, '');
        return domain;
    }
Extracts the domain from a given URL for use in the ?ref= parameter
e.g. https://example.com/ -> example.com
e.g. https://example.ghost.io/ -> example.ghost.io
e.g. https://www.example.com/ -> example.com

@param {URL} url to extract domain from
@returns {string} just the domain, e.g. example.com or example.ghost.io, but not www.example.com