Feature request: Offline Progressive Web App

**Is your feature request related to a problem? Please describe.**

I'm always frustrated when I can't use CyberChef on my phone without an internet connection.

**Describe the solution you'd like**

CyberChef to be a [Progressive Web App](https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Offline_Service_workers) with a [service worker](https://developers.google.com/web/fundamentals/primers/service-workers) so that it will work offline. This just needs a manifest and a script that caches everything offline.

This would allow CyberChef to:
- Be refreshed even after the internet is disconnected
- Be launched easily just by going to https://gchq.github.io/CyberChef/ , no matter if there is internet or not
- Have the browser's "Add to Home Screen" button add it to the home screen like an app rather than a shortcut to a webpage

Examples of PWAs:
- https://angular.io/
- https://regex101.com/

**Describe alternatives you've considered**

The "Download CyberChef" button isn't useful for me. I can't conveniently open ZIPs or HTML pages on my phone.