A: New URL scheme

This turns ```#storage-details?type=drive?id=Foo?machine=bar``` into ```#/bar/storage-details?type=drive?id=Foo```.

This is turned into ```#/bar/storage/drive/Foo``` when Storage becomes a external page in an iframe.

The next step is to remove the need for PageExternal.

Yes.  It's only public here so that both cockpit.js and cockpit-main.js can use it.  The real API comes later, in #1393 or similar.

I'll add tests.