Web Platform Tests on Edge and Safari

I have just merged the final PR in the process of getting
web-platform-tests PRs to run in Edge and Safari via Sauce Labs.

You should start to see PRs reporting results for Edge 14.14393 and Safari
10.0 alongside the existing Firefox and Chrome reports.

In the short term, this means additional comments added to PRs. Hopefully
the benefit of test results in additional commonly-used browsers outweighs
that cost.

In the long term, I am working on a solution that will put all lint and
test results into a single digest comment that is updated as results
change, but will still maintain detailed results logs separate from GitHub.

Thanks to James Graham, Mike Smith, Philippe Le Hegaret, Jeff Carpenter,
and anyone else I may have forgotten for their input and assistance in
getting this done.