Post data to the Aphlict server in JSON encoded form.

Summary:
Ref T4324. Currently, notifications data is `POST`ed to the Aphlict server in the `application/x-www-form-urlencoded` format. This works fine for simple data but is problematic for nested data. For example:

```lang=php
array(
  'data' => array(
    'key'  => '6021329908492455737',
    'type' => 'PhabricatorNotificationAdHocFeedStory',
  ),
  'subscribers' => array(
    'PHID-USER-y7ofqm276ejs62yqghge',
  ),
);
```

Is encoded as `data%5Bkey%5D=6021329908492455737&data%5Btype%5D=PhabricatorNotificationAdHocFeedStory&subscribers%5B0%5D=PHID-USER-y7ofqm276ejs62yqghge`. This string is then (incorrectly) decoded by `querystring.parse` as:

```lang=javascript
> querystring.parse('data%5Bkey%5D=6021329908492455737&data%5Btype%5D=PhabricatorNotificationAdHocFeedStory&subscribers%5B0%5D=PHID-USER-y7ofqm276ejs62yqghge');
{ 'data[key]': '6021329908492455737',
  'data[type]': 'PhabricatorNotificationAdHocFeedStory',
  'subscribers[0]': 'PHID-USER-y7ofqm276ejs62yqghge' }
```

Test Plan: Sent test notifications from `/notification/status/` and verified that the notifications still worked.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4324

Differential Revision: https://secure.phabricator.com/D9386