Add a info_requests_visible_classified_count column to PublicBody

This counts only those info requests that have prominence 'normal'
(i.e. are not hidden) and are not 'awaiting_description' (i.e. that
they have had some basic status classification).
Add "(n)" to the tooltips on the stats graphs
Update the calculation of the numerator for percentage statistics

We have changed the denominator of the proportion-based statistics
to only include requests that are both visible and not
'awaiting_description'.  This would mean, however, that the numerator
could be larger than the denominator.  This commit updates the
calculation of those statistics to also exclude any hidden or
unclassified requests.
Fix the indentation of a PublicBodyController test
Factor out code for generating graphable data and add tests

This simplifies the statistics action of the PublicBodyController
and makes it easier to test the functionality now in the
simplify_stats_for_graphs method.