Support iOS Files

[dakira]

If the new Files would be supported, you'd automatically have support for all major cloud storage providers,

• Dropbox,
• One Drive
• Box
• iCloud
• Google Drive
• Nextcloud
• Seafile

as well as local storage.

Our organization is not allowing the usage of Dropbox, so we're out of luck syncing Boostnote to our mobile clients. Supporting Files should be quite easy.

This would also close several other issues like #94.