R1:3202f0f23ddf

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…