Make it possible to use graphiql without CDN

[mike-marcacci]

This is basically identical to this old issue over at the official graphiql repo: graphiql needs to work in offline/intranet/sandboxed/whatever environments.

I'm currently in a mostly offline scenario (dial-up quality cell service) and the dependency on jsdelivr.net was definitely something I didn't anticipate when getting everything ready for offline work.

[helfer]

Hi @mike-marcacci! 👋 I totally agree, this is a duplicate of #295 I think. A PR would be very much appreciated!

[mike-marcacci]

Agreed! Sorry I missed that one (was quite bandwidth-limited while searching existing issues). I'll see if I can get a moment to help w/ a PR. Just wanted to make sure it was recorded somewhere!