Contributor: Should be able to have multiple roles assigned

[ValentinFutterer]

Describe the bug
Up until now, DAMAp assumed that every contributor could only have one role assigned, but this is not the case in the RDA-Common-standard. Change the data model to allow multiple roles per contributor. Take care of handling old entries with only one role assigned. There could also be a bunch of export related bugs spawning out of this. Take care to work through the export code and check that this doesnt break anything. Talk to Christiane about how to handle different edge-cases, should they come up.

Is this a regression?
No.

Expected behavior
Multiple roles should be allowed and work without errors.

Environment

• Damap version 4.3.0

Frontend Issue