Fixing the User model.

ManagedUser class is gone.  User class now holds all non-enterprise
stuff.  EnterpriseUser now holds the six enterprise-specific properties.
All user management methods now require an EnterpriseUser instance, and
no longer accept custom fields lists:  you get all the data, all the
time.  This will make responses larger, but it will ensure that no data
is casually lost when modifying users.