Story

AS A participant/supporter for the Transplant Games,
I WANT to be able to register for the Games using an intuitive, multi-step form,
SO THAT I can save my progress and return to the form later before submitting.

Details

Need to create an API endpoint to save and update registration details at event_registration/registration.
Requires User model from #11 + #41, Registration model #39 and Registrant Details model #14.

Acceptance criteria

• Create endpoints for registration that creates or updates registration form (PUT)
• They also should be able to continue from a partially completed registration form (GET)