Cannot fill in event fields when registering

Introduced in !477 (merged)

One-sentence description

Cannot fill in event fields when registering for an event with registration fields.

Current behaviour

When you register for an event with fields, you must fill in the fields and press submit. This does not save the fields, but you are registered.

Expected behaviour

The fields are saved and you are registered.

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