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.