Refresh: use variable fonts

[craigcook]

Description

Some designs use a mix of several font weights, which ordinarily means loading a separate font file for each weight and style. These add up quickly so we've always tried to limit the number of font files to optimize performance. But the new brand fonts come in variable versions, and browser support these days is pretty good, so we're thinking it's probably time. The new variable font files are also surprisingly small so it can actually be a performance gain.

We could fall back to the static fonts but then what would be the point, so we're thinking we just... don't. Let older browsers get system fonts. We'll want to do some thorough testing of the fallback state to make sure it still looks good, and we can work out a good font stack with common system fonts.