Use rems for everything instead of pixels

Follow-up to the discussion in !684 (comment 200806252)

All spacing utility classes use pixels, based on Pajamas’ spacing specifications that use an 8px grid spacing system. However, using pixels doesn't allow elements and spacing to scale correctly if the user adjusts their browser zoom level (see !684 (comment 201375954))

Use rems for everything instead of pixels fixes this.

The 8px spacing system from Pajamas uses pixels instead of rems because it's easier for everyone to understand the concept that way, and also because UI design software uses pixels. It would be harder to explain it if we said “We use 0.5rem as the base for determining our spacial system.” as this doesn't mean anything if we don't know the root size, so we would need to add “The root size is 16px”.

When translating designs to the browser, the expectation is that 16px in the design translates to 16px rendered in the browser, but that doesn't mean we can't use rems to specify it. So the current spacing utility classes implementation took a literal approach to the 8px spacing system.