Use core-js URL polyfill instead of whatwg-url

Created by: felixfbecker

whatwg-url has a very large bundle size, because it is basically the pure-JS reference implementation, including a lot of details with domain encoding, punycode, etc. It's not intended as polyfill.

This reduces our bundle size by 28% (0.692MB -> 0.499MB, Δ ~200KB)

Verified that core-js URL parses out hostnames properly from custom hosts (git://), which is the primary thing we care about.