Give the option to delete media from disk when emptying the Media Recycle Bin

The Problem
When the Media Recyle Bin is emptied the generated folders and files remain on disk. Over time this can cause unnecessary calls on developer time to sort out. It's particularly onerous on shared hosting or VM's with limited resources.

From a users perspective the Delete convention has two parts:-

1. The Delete action moves items into a Recycle Bin which can be restored.
2. Once the Recycle Bin is emptied files are physically deleted and no longer available for restore.
   The current situation breaks this user convention and causes unnecessary disk use and client confusion/disappointment.

A Proposed Solution
Add a checkbox to the "Empty recycle bin" dialogue to modify the default action to physically delete the relevant files and folders from disk. (Which is what the current dialogue says anyway).