Create API endpoint for registration form

# 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)