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.
Hmm. The form will send a 0 if not. Although this function will be used by the API too. Would you agree its best practice to specifically check for whether or not the data sent contains a value of 1 sent for delete?