Fix next url in events registrations admin

Ghost User
2018-11-02T15:46:26+01:00

Closes #735

#### Previous behaviour
Steps to reproduce:

1. Create a registration in the backend for and event using 'save and add another'
1. Get the overview instead of form

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

1. Create a registration in the backend for and event using 'save and add another'
1. Get the form