Migrate our service worker implementation to workbox

[julienw]

Currently we use offline-plugin, but it's not updated for 2 years, it looks unmaintained, and from our work in #3124 we discovered it doesn't support Webpack v5: fails with:

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined
    at validateString (internal/validators.js:120:11)
    at Object.resolve (path.js:980:7)
    at afterResolveFn (/home/circleci/project/node_modules/offline-plugin/lib/index.js:213:42)

Therefore we should migrate to workbox.

Workbox has some webpack plugin to find all files handled by webpack. I'm not sure this is strictly needed, but maybe it is.

We need to keep this behavior:

• cache the main html file, as well as all js css and image files
• /docs/* and /photon/* shouldn't be cached and should be returned unchanged
• all requests to other paths should return the same response as for /
• _headers and _redirects are netlify-specific and need to be excluded as well

We also need to rework the behavior as used by ServiceWorkerManager.js. It's pretty well self-contained, so this should be doable.