Refactor HTMLActuator to properly render variant tiles >1000.

The motivation for this patch is that I want to add a variant
merge rule that generates four-digit numbers that aren't powers of
two. The old code styles such numbers with too big a font size;
e.g., a tile with value 1234 would be styled with class "tile-1234",
which doesn't exist, and then it would render sloppily. I believe
a friendly default behavior is to style such tiles with "tile-1024".

I'm also leaving the door open for variants to style the color of
the tile and its font size using two (or more) separate CSS classes:
e.g., ["tile-red","tile-4digits"].

No intended functional change.