Add support for Use Small Fonts.

Reduces postbody size to 80%, and adjusts some other margins and sizes to make
a more compact display. No effect on other views yet.
actually use the full small-font CSS
Awful.useSmallFonts -> Awful.setFontScale; add basic font-scale setting.
Update table row when font-scale stepper changes so that label recomputes.
Remove all useSmallFonts stuff, actually save fontScale setting.
Add font-scale stepper to page settings. (Not wired yet.)
Wire up stepper to settings and label-update.