Don't distribute our html guide in the tarball
shell: Simplify realms code for System Information page

There were some leftovers from when the dedicated realms page was
removed.
shell: Remove getTitle method of dialogs.

It is not used and just duplicates what is alread in the
h4.modal-title of the dialog.
shell: Factor out common code into get_page_machine.
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.