Templates: improve theme compatibility for how we set small font sizes.

The change is best demonstrated by the TwentyFifteen theme, which uses the HTML body font size reduction technique to facilitate easy calculations for setting REM/EM font sizes; e.g. 1 == 10px, 1.4 == 14px. Whwn we use a REM size < 1, we're reducing these font sizes even further, and can sometime get too small and hinder legibility (specifically for the `.button` element).

Using `small` is our current best idea for smaller font sizes across all different types of themes / font sizing techniques.

Fixes #6022, props hnla

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9279 cdf35c40-ae34-48e0-9cc9-0c9da1808c22