Registers the device's FCM token for the current user, or refreshes it if it is already registered. Once registered, the device receives the push notifications of the events the user has subscribed to.

Call it at every app start or login, and whenever Firebase rotates the token.

• Always send user_agent and platform: a field left out on a later call is reset to null.
• label is applied only on the first registration. To change it, or to disable the device, use Update push device.
• iOS apps must get the token from the Firebase Messaging SDK: a raw APNs token is not accepted.

On logout call Unregister push device.