R1:fe98aa6839f0

Publish additional context to the Aphlict server.

Summary:
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…