Fix filling in event fields when registering

Closes #465 (closed)

Previous behaviour

Steps to reproduce:

1. Go to an event with fields
2. Make sure you are not registered
3. Press the register button, your registration should be submitted
4. Fill in the required fields
5. Get redirected to the events page, with the error "you were already registered"
6. Press 'update registration'
7. See that your submitted fields have not been saved

New behaviour

Steps to validate that it works:

1. Go to an event with fields
2. Make sure you are not registered
3. Press the register button, your registration should be submitted
4. Fill in the required fields
5. Get redirected to the events page, with the error "you were already registered"
6. Press 'update registration'
7. See that your submitted fields have been saved