Use current script URL when connecting to WS server.

Created by: n6g7

Hey,

In this PR I added support for connecting to a WS server on a different origin than the one of the current document (useful in cases where the app HTML isn't served by the CRA dev server). See https://github.com/facebook/create-react-app/issues/4468 for more info.

To do this I used the same approach as WDS: instead of using window.location in every case, try to use the url of the current script.

To test this feature:

• Test it's still working in the simple case:
  • Run yarn start
  • Open http://localhost:3000 in your browser
  • Edit packages/react-scripts/template/src/App.js and save
  • See the page hot-reload, as before.
• Test it's working when serving the HTML from a different port:
  • Run yarn start
  • Create a temporary directory: mkdir tmp
  • Get a copy of the app's HTML with links to the CRA dev server: curl localhost:3000 | sed s_=\"/_=\"http://localhost:3000/_g > index.html
  • Run a second test server locally: python -m http.server 8000
  • Open http://localhost:8000 in your browser
  • Edit packages/react-scripts/template/src/App.js and save
  • See the page hot-reload, as before
    • It might be blank after the reload, that's because the *.hot-update.js part of the html is out of date, re-running the curl command above fixes it.