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).
For percentage stats, exclude hidden or unclassified requests

The WDTK volunteers pointed out that it's not fair to include
hidden requests in the denominator, since they're typically hidden
for a good reason (e.g. being vexatious, spam, etc.), and we have
no information about those that are awaiting_description (i.e.
unclassified) so they should be excluded as well.
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.
Don't include public bodies tagged 'test' in public body statistics

In the initial release of public body statistics to WhatDoTheyKnow
a public body only intended for testing ("mySociety Test Quango")
was included in the statistics.  This commit causes public bodies
tagged with "test" to be excluded from the public body statistics
page.

Fixes #1115.