[development] Implementing a user deletion policy

[Steve Yelvington]

Thanks, Earnie & Earl.

Unfortunately, in pre-D7 and D7 cases the user hooks delete the user 
from the database before invoking any additional modules, so there 
doesn't seem to be an easy way to implement a global no-deletions policy.

I suppose I'll have to chase down and filter all of the forms that might 
implement a deletion option.