and asking for a user from a user database given an ID.  The default
implementations, in BasicUser and BasicUserFolder assume that user
names are used as IDs.

Refactored the user classes so that super and nobody aren't persistent
and don't have IDs. There is now a SimpleUser that is the same as
User, sans persistence.  There is a SpecialUser that subclasses
SimpleUser.  The special user nobody is a SpecialUser. The class Super
not subclasses SpecialUser.