List each member only once in Registration admin

Closes #484 (closed)

Previous behaviour

Steps to reproduce:

1. Create a Member with multiple Memberships
2. Try to create a Registration through the admin for the Member created in the previous step
3. Notice that the Member appeared multiple times in the list, and that the Registration creation failed.

New behaviour

Steps to validate that it works:

1. Create a Member with multiple Memberships
2. Try to create a Registration through the admin for the Member created in the previous step
3. It works