Synchronization: Explore cloud messaging for providing real-time syncs

[mtotschnig]

Currently when data is written to the back-end on the first device, the second device receives this data either when its periodic sync runs, or when user manually calls Sync now menu option.
By using an optional cloud messaging service, e.g. FCM, the user experience could be improved at the cost of the risks to privacy inherent in cloud messaging.