[development] Implementing a user deletion policy

[Steve Yelvington]

We ran into a problem involving deleted accounts, so I'm looking into 
implementing some tighter controls. However, it doesn't appear that 
user_delete() is a hook, so I can't intercept it. I suppose I can filter 
out the "delete" button, but are there any other obvious solutions? I 
don't want to kill any kittens.

Also, Watchdog logs the deletion but not the ID of the user who deleted 
the account. Seems like an oversight.