Switch the user version of the cluster status page to a new page that
provides a table of immediately available nodes as well as the time
series graphs. Note that the graphs are slightly different on this page
since we are shoiwing "available for allocation" instead of "available
to reserve", so we have to add the 'held' number to the free count
returned by Forecast().

The bulk of the commit is really shuffling things around a bit to make
it easier to reuse the graphs code in the new page.