Add a page with statistics on public bodies

Stastistics like "how many successful requests were made to
The Department of Metasyntax?" are too slow to calculate
on-the-fly for this page, so this commit adds:

 * Extra columns on public_bodies to store summary statistics

 * A rake task which should be run periodically to update
   those statistics columns.  (This will also update the
   existing info_request_count column, which it seems wasn't
   being updated, except at the point where the migration
   introduced it.)

If Javascript is not available, the summary statistics are
shown as tables.  If Javascript is available, graphs are
drawn with Flot.