Size tag font depending on popularity (wincent.dev, b724d1a)

According to "Web 2.0" doctrine you need tags and you need to change the font size to indicate the popularity of a tag.

This commit introduces the resizing in the range of 1.0em (for impopular tags) through to 2.0em (for the most popular).

Signed-off-by: Greg Hurrell <greg@hurrell.net>