As in #7284

, it can often be useful to style text like a heading in
places where using the heading tag would be either semantically
incorrect, less accessible, or otherwise invalid.

Support this in .jumbotron as in the rest of the document by allowing
elements with class .h1 to stand in for the h1 element.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

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