Use FCM topics to subscribe to a device alerts

[szimek]

We could subscribe to devices in general or to a specific detector on this device (e.g. /devices/:device-id or /devices/:device-id/:detector). This way we can send only one notification and FCM will send it to all subscribed devices.

This info needs to be stored in both FCM and Firebase db, so that we can show state of each subscription.

More info how to subscribe a device to a topic: https://developers.google.com/instance-id/reference/server#create_relationship_maps_for_app_instances (Instance ID is the token we get from Firebase on the client).