Infra: Use Workbox v5

[fpapado]

Workbox v5 (the webpack plugins, in particular) is in alpha 🎉
Alpha Release Notes

This gives us a few goodies:

The Service Worker script is now bundled with webpack (as a child compilation). We can use import statements directly, that get inlined by webpack. This skips the injectScripts auto-magic imports (and avoids some footguns). It is slightly smaller because of tree-shaking. I also find it more explicit and readable!

The manifest is now inlined in the Service Worker script. One less thing to remember in setting up headers, and also reads a bit better imo.

We can use dontCacheBustURLsMatching to avoid revisioning assets with [contenthash]. This allows the Service Worker to use the HTTP cache when precaching, which should yield some bandwidth savings, and maybe whatever load there is.

Plan

Let's keep this running in deploy preview for now. I don't see any huge or breaking changes apart from the plugin so far. Might be good to send the Workbox team a message about the update experience!

Some brief notes:

• In v4, a call to importScripts(/* some local version of wb.js */) would be added, with injectManifest. Now it isn't. This is ok, though I think the docs could be more explicit about "you know have to import things directly". It is described more as a "potential" than a "need" atm, which is not accurate.

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