Listing captions to activecodes

[ascholerChemeketa]

This implements the "Runestone activecode steals caption of listing it is in" so that explaining text is placed inside RS box.

@rbeezer It also moves listing caption above the listing in HTML output to match CMOS style. I don't trust my understanding of the LaTeX to update that output path. We could break this out separate, but it does improve the readability of listings, which benefit from some context before the reader hits a wall of text.