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.

(Hmm, of course, get_page_host will get changed as well, so mayb cockpit.js doesn't need decode_page_hash.)