Sync: Provide a way to remove remote data

[palant]

As mentioned in #86, broken remote data might cause sync to be stuck - uploads always result in an error. The recommended solution is removing remote data, an option that might be difficult to find (Google Drive) or even impossible (5apps via remoteStorage). It would be better if PfP could offer removing the data itself for sync_tampered_data, sync_unrelated_client, sync_unknown_data_format errors.

Optional but nice to have: optionally delete remote data when disabling sync.