Add support for offline placeholder images

[tgerulaitis]

We should add a placeholder image to return from the service worker when an image request fails (e.g. when offline and the image hasn't been cached). Also add a system configuration option to override the placeholder image with a custom one for the store.

[thomas-thaulow]

Did this ever get started on?

[tgerulaitis]

@CiaraCode, nope, we're not doing any active development on this extension at the moment. Feel free to pick it up if you wish!

Cheers,
Tomas