Flasher re-downloads and unpacks same images

[incognitojam]

After flashing a device I refreshed the page to test again and it re-downloaded and started unpacking the images again. The storage quota used was increasing in devtools.

Need to make sure:

• if the image checksum fails, to delete the image/archive
• delete old images?

[adeebshihadeh]

I think we delete the old images. We need to keep the memory usage super minimal, and I think we'll never even see the full image -- just stream the chunks directly to the device as they come in.