Two registrations one email address

Ghost User
2018-09-26T20:28:22+02:00

Closes #691

#### Previous behaviour
Steps to reproduce:

1. Create registration without student number
2. Create registration with student number, same other info
3. Works

#### New behaviour
Steps to validate that it works:

1. Create registration without student number
2. Create registration with student number, same other info
3. Fails!