Prepare for guest migration

Introduces the new identity type migration id, meant to be used when
migrating a person from another system.

The cerebrum guest import script has been expanded to use this new
migration id. The script has also gotten an option to import people
without the necessary IDs (NIN/Passport) so that we can generate IDs for
them in GREG. The can then be imported by cerebrum manually before we
add the remaning info from a different source. In this case we use old
info from UiO SAP.