Tidy up anonymous feedback index

* remove redundant heading
* display nicer message when no feedback is found:

![image](https://cloud.githubusercontent.com/assets/23801/3810291/2d35cfba-1c8e-11e4-9b44-a3885d442271.png)

/cc @fofr 
While we're here, can we add a `table-header` class?