Tidy up anonymous feedback index

* remove redundant heading
* display nicer message when no feedback is found:

![image](https://cloud.githubusercontent.com/assets/23801/3815184/29b8b8b4-1cc2-11e4-986f-90540c48e0cc.png)
* switching header over to use the `govuk_admin_template` styles

/cc @fofr