web: Don't omit "dashboard" from URL.

This allows people to explicitly omit the dashboard ("All") from the
breadcrumb trail, such as when embedding Cockpit into a UI that
already takes care of multiple machines.

In addition, the "Account Settings" and "Search" widgets use te new
cockpit_go_server function to do their navigation.  That function
makes sure that the new breadcrumb only includes "All" when it was
there before.

Fixes #465.