Investigate caching issues

[RReverser]

Looks like whenever we update CSS, it's not properly updated right away on clients - either due to Chrome's own aggressive caching, or our usage of Service Workers.

This leads to issues like this: https://twitter.com/RReverser/status/1397952567213268994.

/cc @mathiasbynens in case you have quick ideas

[mathiasbynens]

Maybe we should cache-bust by serving main.hash.css instead of main.css?

[mathiasbynens]

(That's assuming the problem is Chrome's own caching rather than the service worker — I didn’t check!)