Ref T4324. As well as sending the key for the notification, also publish the notification type and a list of subscribers to the Aphlict server.
The idea here is that the Aphlict server passes anything within the data key to the clients, whereas other keys (such as subscribers) will be used by the server to determine where the notifications should be routed.
Note that these changes don't do anything useful, but are a prerequisite for further work on T4324.