Switch to CDNs for JS library files

[MichaelCurrie]

This will speed up the loading of the site.

https://blog.stackpath.com/glossary/cdn/

[cheelee]

Just a quick update - I've moved all external dependencies for js libraries to webworm/static/extern/dist. If they are replaced by CDNs, we just need to make sure we remove the .js files from the extern folder.