Bug 7642 - [3.8.x] fix the lost display of tag sizes

Bug 7642 - [3.8.x] fix the lost display of tag sizes

This fixes the display of different sized words for differently popular
tags. It is a bit of a refactor of that part of the system, moving
logic to more sensible places (and removing an unused method on the
way.)

Note that it isn't an attempt to reproduce what was there previously,
just to do something similar, and in an easier to change fashion.

Sponsored-By: New Zealand Educational Institute
Signed-off-by: Marc Veron <veron@veron.ch>
Works as expected.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>