Add service worker and offline page

[greatislander]

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

In order to perform optimally as a PWA, we need to implement a service worker.

Describe the solution you'd like

The service worker should cache CSS, JS, and SVG assets, and should redirect users to a cached offline page if their connection is interrupted.

Describe alternatives you've considered

Not applicable.

Additional context

Not applicable.