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>

.eslintrc

@@ -19,7 +19,6 @@ globals:
   CodeMirror: false
   Dropzone: false
   emojify: false
-  hljs: false
   SimpleMDE: false
   u2fApi: false
   Vue: false