Switch all sizes (except borders) to 'rem' units.

'rem' doesn't have a "fixed scaling" ratio (such as 100%, 150% or 200%) on
mobile browsers, and is instead much more proportional.
Allow thumbnail images to be scaled.

This will allow us to set the thumbnail size to a decent viewport fraction
instead of relying on the effective thumbnail size (much like what we already
do with the main image).

For now, set it to a 1:1 effective pixel ratio, which fixes shitty scaling on
mobile browsers without having to rely on the "initial-scale" meta-tag.