Update to auto-labeling of pull requests on GH

Hi folks,

Just a quick update to let you know I've pulled and deployed some changes
from Dom which now automatically tags pull requests with their related
working group(s).

I ran the script against pre-existing pull requests but have run into
issues in doing so, so I'm not sure whether I caught all of them or not. If
you bump into a pull request which is missing the correct tags, please add
them manually. Thanks.

In any case, this allows you to watch all open pull requests against a
particular WG like so:

https://github.com/w3c/web-platform-tests/issues?labels=wg-webappsec

Of course, if you want to dig deeper, you can still get all PR for a
particular spec by using it's shortname:

https://github.com/w3c/web-platform-tests/issues?labels=cors

Hope this helps.

--tobie