Read position sync via iCloud

[chucker]

Requested here: https://balkan.fedive.rs/@mattmarenic/110175571581639109

• [ ] Evaluate if this is possible with notarization at all
• [x] Make UI that switches between this and Mastodon API sync (see also #37)
• [ ] Abstract #37 to have two different sync providers

[chucker]

(Tried with CloudKit, but am now thinking NSUbiquitousKeyValueStore is more appropriate.

Also, do we need to check for iCloud? Probably.

Do we also need check if our app is enabled in iCloud? Unclear.)