[w3c/ServiceWorker] Route showNotification() URLs through a fetch event? (#1161)

Right now (on Firefox and Chrome, at least) any `image`, `icon` and `badge` URL passed into showNotification() goes straight to the remote URL. It would be a much better experience if those requests were routed through the fetch event, so that we could use cached assets instead.

There is a work-around using `cache.match()` -> `res.blob()` -> `FileReader.readAsDataURL()`, but it's pretty inefficient since the lack of `URL.createObjectURL` (as removed in #688) means that every image is turned into a base-64 encoded string. Since the introduction of the `image` attribute, some of these images can be pretty huge.

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https://github.com/w3c/ServiceWorker/issues/1161