Add some more navigation and details to parent show pages

e.g. on an org show page list the sites, on a site show page list the hosts.
Show only the most recent hits, and a progress bar
Extract most recent hits section to partial

Also the data collection to an object.  We render a simple stacked chart by status and a broken down chart for each hit.
Lazily evaluate calculations in most recent hit data object
Allow for hits to be aggregated

This allows us to show the aggregated hits for a site (e.g. all hosts).  Have to use some gnarly ARel stuff to construct "select calculations from (subquery)" style data, but it seems to work.