Fix bugs with exporting Rubric CSV

Fixes #671 (closed)

Fixes #696 (closed)

CSVs now have "external:SomeIdentifier" instead of a netid if a user does not have a netid.

Project ID,Assignment Naam,Gebruiker Name,Gebruiker NetID,Rol
54,internshsip one,David Alderliesten,dalderliesten,student
54,internshsip one,Maurício Aniche,maniche,Supervisor
54,internshsip one,Grzegorz Brzęczyszczykiewicz (Unregistered),external::GrzegorzBrzczyszczykiewiczUnregistered,Supervisor
54,internshsip one,Test Company1,external::Test Company1,offerer
Edited by Martin Mladenov

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