Navigate: remove "gone async" and define redirect handling

Redirect handling had the following issues that are now fixed:

* /a navigating to /b which redirects to /a#test would not unload /a.
* Headers set on the initial request were not preserved.
* Redirects to javascript URLs would not result in an error.
  Fixes #314.
* Location header was not parsed, and HTTP method was not changed
  where needed. Fixes #461.

Another source of complexity was the "gone async" mechanism, which
was used for two steps that could just as easily be factored out and
had to be since they were run synchronously, but should not be.

Changing that fixes #1446 which discusses the issue around unknown
URL schemes in more detail.