Fix race condition in registration approve/save

Ghost User
2019-04-03T20:32:13+02:00

Closes #841

#### Previous behaviour
Steps to reproduce:

1. Create new registration
2. Try to approve + save within couple of seconds
3. Empty payment

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

1. Create new registration
2. Try to approve + save within couple of seconds
3. Cannot save + existing payment