shell: Don't encode breadcrumb in URL.

Instead, show a global breadcrumb that always looks like

  Hosts >> $hostname >> $page

The hierarchical navigation functions "go_up", "go_down", and
"go_sibling" have been replaced with "go_rel".

Every page now has a "machine" option that gives the address of the
machine to connect to.

This is an intermediate step, and the next commit restores full
navigation by putting local breadcrumbs into the individual pages that
need them.
shell: Local breadcrumbs.
shell: New layout for navigation.
shell: Adapt update_global_nav to new layout.
shell: Move system information code to server page.
shell: Remove "Rescue Terminal" option from dashbord menu.
shell: Highlight active navigation tab.
shell: Save space on the server dashboard

By using the hostname and realms directly as the label of buttons for
changing them.  Also, always leave a realm via a dialog.
test: Adapt check-shutdown-restart