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.
Discussed with @sergii-tkachenko -- since the Cancel button doesn't actually uncheck the checkbox yet, going to keep as `alert` for now.