[WIP] Cache / Download Images

[agilgur5]

Still getting bugs with react-native-expo-image-cache, even on my fork. Might be a Babel 7 thing.

This is still not ideal as caching is a bit different than the standard reader app functionality of "download / save for later". Long-term storage and unnecessary storage are issues. The other issue is that at least some sites have signed/timestamped images; if they're cached, they won't expire by timestamp of course, but retrieving the image might not hit the cache.

This likely needs a more custom solution with FilesystemStorage, which would be way more effort. The timestamp concerns and general restructuring of how pages/images are retrieved also requires more integrations with different sites to see how different ones do it.