Auto-deploy/publish to gh-pages w/ GitHub Actions

[sideshowbarker]

This is based on the setup Domenic Denicola put together at https://gist.github.com/domenic/ec8b0fc8ab45f39403dd

[annevk]

Can you migrate the Travis integration if there's anything remaining as well?

[sideshowbarker]

Can you migrate the Travis integration if there's anything remaining as well?

I can do that in a separate follow-up. To be clear, that amounts to just being some stuff that runs Bikeshed for PR branches. But for now we can just keep it.

There are some other follow-up changes we can make after merging and testing this — for example, removing the index.html file from version control.

[sideshowbarker]

OK, for the record here, this all seems to have worked:

• 5de99e4
• https://github.com/w3c/webappsec-csp/runs/1934313987
• https://w3c.github.io/webappsec-csp/ has today’s date