Don't register users for XMPP upon creation.

- The IPrincipalCreatedEvent is called BEFORE a user's memberdata properties are
set. This makes this event useless if we also want to set a VCard with the
user's memberdata.
- Users can now be automatically registered for XMPP upon next logged in visit,
  or manually via the XMPP user setup page (@@xmpp-user-setup).
- In both these cases, memberdata is already properly set.
Update help strings for the XMPP controlpanel configlets
Update HISTORY
Remove unused import