add a site/authority x user/person relationship in registry model for

keeping track of PIs
plimporter maintains this as it can (with the obvious limitation of
the plcapi model where persons receive PI role for *all* their sites)
--- still todo
. adapt native sfa registration for dealing with PIs
. tweak determine_user_rights to use native registry info rather than
  from augment_records_with_testbed_info stuff