Add the ability for any spec to have a service worker

[domenic]

Part of whatwg/meta#3. This allows any standard using the shared deploy script to automatically get a service worker generated, which delegates all of its logic to a shared file on resources.whatwg.org. It allows room for future expansions of the deploy script to include extra resources.

This still requires registering the generated service worker in each spec's HTML, however.

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On top of #52. To get this working on specs using the new deploy script (e.g. URL), include the following at the bottom of your page:

<script>
"use strict";
if ("serviceWorker" in navigator) {
  navigator.serviceWorker.register("/service-worker.js");
}
</script>

We could in the future add that our Bikeshed template, once all specs are on the new deploy script (whatwg/meta#11).

I tested this locally, kinda (without the shortname inference), but as usual it'd be best to merge this and try it on specs when multiple people are online.

[domenic]

@jakearchibald pointed out a potentially deadly problem with this, which is that as implemented now, no browser will ever update this service worker, even if we update standard-service-worker.js. Since the actual service worker is byte-for-byte identical, it doesn't bother revalidating importScripts()-ed scripts like standard-service-worker.js.

This is fixed in the spec but not implementations.

We could work around this in the future by writing out a "version" comment in the deploy script if necessary, e.g. add a line like // v2. I guess it's OK as long as we remember to do that (for as long as the browsers don't implement importScripts() checking).

[zcorpan]

Could the comment be automated somehow (maybe include commit hash)?

[domenic]

Hmm, that might work... to be clear, it'd be the commit hash of standard-service-worker.js from resources.whatwg.org. I wonder how to get that without too much code...

[domenic]

I've updated this to add a version comment and be rebased on master.

[annevk]

I don't see any obvious problems, but I'm not as familiar with these things as @jakearchibald is. Happy for you to give this a try though.