Clean up generation of structured data

[stadust]

pointercrate places some structured data in the head tags of pages to help google in understanding the pages. As part of this, level descriptions get embedded, which causes all sorts of issues if they contain special characters. Right now, everything just ends up HTML escaped, which works well enough, but seeing " in the description isn't too nice. We should at least JSON-escape it first, and then only HTML escape for safety in case we missed something, so that quotes at least display correctly.