disable Charter web font, switch back to system fonts

I'm basically obeying the arguments in:

https://bt.ht/webfonts/

This reverts parts of commit 0bca0fb7 (switch to the Charter font,
2020-03-10), basically the @font-face bits, while keeping the other
improvements. We do keep a strict max-width, sans UI elements,
italicized titles and other such tweaks.

We also drop detailed UI fonts and stick to the simpler "system-ui"
CSS property, overriding it only for Linux to keep Liberation out of
the list (only as a fallback) as we prefer Noto.