Use user primary key in pizza API

Ghost User
2017-10-14T12:33:55+02:00

#### Previous behaviour
Steps to reproduce:

1. Create multiple users and make sure the member pk is not equal to the user model pk.
2. Create a pizza event and place an order.
3. Notice that the value of 'member' is *not equal* to the user model pk.

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

1. Create multiple users and make sure the member pk is not equal to the user model pk.
2. Create a pizza event and place an order.
3. Notice that the value of 'member' is _equal_ to the user model pk.