Improvements to Add Tracker modal UI (PMT #96026)

Adds the AJAX response to the modal dialog, telling the user that their
form submission succeeded, or failed, and why.

![screenshot from 2014-08-08 17 18 01](https://cloud.githubusercontent.com/assets/59292/3862936/80c0f0a0-1f41-11e4-8447-caeb3c1f6d68.png)

![screenshot from 2014-08-08 17 17 42](https://cloud.githubusercontent.com/assets/59292/3862937/830799ae-1f41-11e4-9549-075b65e8736d.png)