web: New cockpit.location() object for navigating

This will prevent navigation if methods on the resulting object
are called after the user has already navigated.

Signed-off-by: Stef Walter <stefw@redhat.com>
 - Renamed to location and Location
 - Make .navcount an implementation detail, don't expose
web: Use a Location to go up after container operations

Fixes #369.
Closes #638

Signed-off-by: Stef Walter <stefw@redhat.com>
 - Renamed page_navigator to location