Exclude libraries from backup on iOS

Currently, the files in the app sandbox of the app are included in backups on iOS.

This is - so far - bad, as:

1. We promise that a local library never leaves the system.
2. If a device restores from backup and a library belongs to an (online) account, we trigger a (weird) sync, which is not what most users will expect.

We should - for now - completely disable backups on iOS (similar to what we do on Android).