ServiceWorker not kicking in on force-reload

Relevant:

• https://stackoverflow.com/questions/37559415/how-to-make-serviceworker-survive-cache-reset-shiftf5
• https://developers.google.com/web/fundamentals/primers/service-workers/lifecycle#shift-reload

Might require a simple FAQ entry. Unless we want to muck around with how force-reload is handled.