List each member only once in Registration admin
Closes #484 (closed)
Previous behaviour
Steps to reproduce:
- Create a Member with multiple Memberships
- Try to create a Registration through the admin for the Member created in the previous step
- Notice that the Member appeared multiple times in the list, and that the Registration creation failed.
New behaviour
Steps to validate that it works:
- Create a Member with multiple Memberships
- Try to create a Registration through the admin for the Member created in the previous step
- It works