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>

templates/base/footer.tmpl

@@ -24,9 +24,6 @@
 {{end}}
 
 <!-- Third-party libraries -->
-{{if .RequireHighlightJS}}
-	<script src="{{StaticUrlPrefix}}/vendor/plugins/highlight/highlight.pack.js"></script>
-{{end}}
 {{if .RequireMinicolors}}
 	<script src="{{StaticUrlPrefix}}/vendor/plugins/jquery.minicolors/jquery.minicolors.min.js"></script>
 {{end}}