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 notification.

Test Plan:
I opened up two clients on the same host (incognito tabs in Chrome). Here is the output from the server:

```
> sudo ./bin/aphlict debug
Starting Aphlict server in foreground...
Launching server:

    $ 'nodejs' '/usr/src/phabricator/src/applications/aphlict/management/../../../../support/aphlict/server/aphlict_server.js' --port='22280' --admin='22281' --host='localhost' --user='aphlict'

[Wed Jun 11 2014 19:10:27 GMT+0000 (UTC)] Started Server (PID 4546)
[Wed Jun 11 2014 19:10:36 GMT+0000 (UTC)] <FlashPolicy> Policy Request From ::ffff:192.168.1.1
[Wed Jun 11 2014 19:10:37 GMT+0000 (UTC)] <Listener/1> Connected from ::ffff:192.168.1.1
[Wed Jun 11 2014 19:10:37 GMT+0000 (UTC)] <Listener/1> Received data: {"command":"subscribe","data":["PHID-USER-cb5af6p4oepy5tlgqypi"]}
[Wed Jun 11 2014 19:10:37 GMT+0000 (UTC)] <Listener/1> Subscribed to: ["PHID-USER-cb5af6p4oepy5tlgqypi"]
[Wed Jun 11 2014 19:10:39 GMT+0000 (UTC)] <Listener/1> Received data: {"command":"subscribe","data":["PHID-USER-kfohe3ca5oe6ygykmioq"]}
[Wed Jun 11 2014 19:10:39 GMT+0000 (UTC)] <Listener/1> Subscribed to: ["PHID-USER-kfohe3ca5oe6ygykmioq"]
[Wed Jun 11 2014 19:10:42 GMT+0000 (UTC)] notification: {"key":"6023751084283587681","type":"notification","subscribers":["PHID-USER-cb5af6p4oepy5tlgqypi"]}
[Wed Jun 11 2014 19:10:42 GMT+0000 (UTC)] <Listener/1> Wrote Message
```

I verified (using the "Network" tab in Chrome) that an AJAX request to `/notification/individual/` was only made in the tab belonging to the user that triggered the test notification.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5284, T4324

Differential Revision: https://secure.phabricator.com/D9458