Study programme is required during model save of Registration

One-sentence description

Study programme is required during model save of Registration

Current behaviour

A registration should always contain a study programme, even for supporters.

Expected behaviour

Supporters should not be forced to have a study programme in their profiles

Steps to reproduce

  1. Add a new registration in the admin, make sure the type is 'supporter'
  2. Save the registration
  3. Error for the programme field
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