Fixes #9925: convert font data to base64, fixing 0xefbfbd (Unicode Replacement Character) chars in customizer fonts.

Merges #9982

I probably missed a few things and screwed up a few others, but this
takes the place of much of #9910. I haven't added many variables—that
should be reserved for v3.1—but I have rejiggered the layout, added
missing ones, corrected wrong values, etc.

/cc @cvrebert @juthilo @saas786

follow up to #9963 and #10005: update docs to use .screen-*-min vars, update customizer as well; run grunt to adjust raw files