R1:84d259cea223

Modify the Aphlict server to transmit messages instead of broadcasting them.

Summary: Ref T4324. Ref T5284. This adds server-side support for keeping track of a set of PHIDs that the Aphlict clients have subscribed to. Instead of broadcasting a notification to all clients (after which the clients can poll `/notification/individual` in order to determine whether or not they are interested in the notification), transmit notifications only to clients that have subscribed to a PHID that is relevant to the…