Replace Networking with custom caching solution

[skjiisa]

Features that would be good to have in a custom caching solution:

• Better multi-account support for thumbnail fetching.
  • Currently, I believe if two accounts have images with the exact same path, because of the way Nextcloud handles thumbnail fetching, a cached thumbnail from one account will show up for the other account. Fixing this would be great.
• Folder structure for disk cache for easy cleanup.
  • Currently, if an image is deleted off of the WebDAV server, the cached copy of that image will remain on-device indefinitely. Having a folder structure to the cached images would allow cached copies of deleted images to be cleaned up upon listing files.

This would come with at least some changes to the existing files and would need to be a new major version which should include a function to clear the old, now unused cache folder.

[skjiisa]

Another great feature similar to the cleanup upon listing files would be to store images' etags when caching them. Then when listing files, if the etag has changed, it would know to refetch the new image.