move highlight.js to npm/webpack (#10011)

- introduced window.config to help with js-based lazy-loading
- adjusted webpack chunk naming to avoid 'vendors~name.js' that webpack
  defaults to for vendor chunks.
- added theme class to html and prefixed all selectors. this is
  neccesary so that the theme styles win over the lazy-loaded ones.

Co-authored-by: zeripath <art27@cantab.net>

web_src/less/index.less

@@ -13,3 +13,4 @@
 @import "_admin";
 @import "_explore";
 @import "_review";
+@import "~highlight.js/styles/github.css";