Changes a relative URL to an absolute URL.

@example
toAbsoluteURL('hoge.html') // http://example.com/hoge.html

@param {string} url Relative URL
@returns {string} Absolute URL
function toAbsoluteURL(url) {
  const a = document.createElement('a');
  a.setAttribute('href', url); // <a href="hoge.html">
  // @ts-ignore
  return a.cloneNode(false).href; // -> "http://example.com/hoge.html"
}