Re-implement service worker

[flibbertigibbet]

As a result of #1005, caching with the service worker for the site was disabled. The service worker is still present, as it is necessary to support "add to homescreen."

At the time the service worker was added, only Chrome supported its API. Now, all major browsers support service workers.

Add back service worker caching and test the site as a PWA across platforms. For Windows/Edge, this should now work to install the PWA to the Windows app store.

Consider using Workbox to abstract service worker management, as it may be more robust and easier to support.