Delete Reportback Images

Refs #3564 - allows staff to delete a Reportback image from the new review form.  This is necessary for when users upload things we don't want hosted on our server.

Also cleans up the Reportback Files and related `file_managed` Files upon deleting a Reportback.
Maybe it would be better to use [`confirm`](http://www.w3schools.com/jsref/met_win_confirm.asp)?