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.
@aaronschachter and I agreed to assign the confirm dialog to the submit button in future and leave this alert for now.