Fixes for new logarithmic tag cloud display

The logarithmic display is now the only option (the LocalSettings.php
setting was removed), the display is now correct for subcategories as well
(before, it was just for filter values), and handling was fixed if there's
no difference between the lowest and highest number of values.

Change-Id: Ic6a7f38063d60b1b611a263a8e7f2a27e9f494a8