ConstraintViolation Session.start can not be null

[aslakknutsen]

There is a miss match somewhere when letting Hibernate generate the database schema from current source.

Session.start is nullable false:
https://github.com/onslyde/server/blob/master/src/main/java/com/onslyde/domain/Session.java#L95

While during first user Member Registration and Session creation a Session is created for the User without any start date and cause a 'constraint validation error':
https://github.com/onslyde/server/blob/master/src/main/java/com/onslyde/service/MemberRegistration.java#L52