allowedUrl = function (url) {
    url = url.toString().replace(/['"]+/g, '');
    if (/^https?:\/\//.test(url) || /^\//.test(url)) {
        return url;
    }
}
Check if URL is allowed
URLs are allowed if they start with http://, https://, or /.
NOTE: # urls are not allowed as clicking them will break the editor when clicked