Implement iCloud syncing

[keeshux]

• https://developer.apple.com/documentation/coredata/nspersistentcloudkitcontainer/3618808-eventchangednotification
• https://developer.apple.com/documentation/coredata/nspersistentcloudkitcontainer/eventtype/import

Toggle iCloud sharing on a profile basis. Two Core Data stores would therefore coexist, profiles would belong to local store or local + iCloud.

Remember to deploy container to production eventually.

Closes #555

[keeshux]

Could use a "hidden" iCloud store only to send/receive updates, while always acting on a local store.

Sort of how git works.

[keeshux]

On entity rename in CloudKit store:

Cannot migrate store in-place: CloudKit integration forbids renaming 'CDProfile' to 'CDProfileV3'. Older devices can't process the new records

No problem with migrating local store.