[Cross-device sync] Save downloaded firebase items locally so they're available offline

[glennhartmann]

Similar to #102 - simple and straightforward, no sign-in, auth, etc. Just download images from the storage bucket.

[glennhartmann]

I had forgotten why this was still open, so pasting in context from #108 :

I say "contributes to" and not "fixes" because although we display the images on the page, we don't actually save them into our local storage yet (ie, if you go offline, they disappear).