A way to immediately unregister a service worker

Spin off from #468.
Alex:

I a conversation I had with @metromoxie makes me think we should also add a special none or blank
value to Service-Worker-Allowed to disable SWs entirely. Many users want a similar kill-switch.

I'd be happy with a different header to kill things, but something seems necessary.

Anne:

I had the impression unregister() was the way to remove a single service worker and there would be no protocol equivalent.

If we want a protocol solution to removing a service worker, using HTTP 410 seems much more applicable.

Using none as keyword to forbid service workers of any kind seems fine although it's a bit unclear to me how you disambiguate that from a relative URL. Would we require URLs in that header to start with a slash?

Start with the discussion in #468 and this issue's comments, focusing on Service-Worker-Allowed, unregister(), and the proposed HTTP 410 behavior. The issue has not settled whether a header or protocol response should provide the kill-switch; work is done only after the semantics and specification changes are agreed.