SPIKE/POC - CSR-137 multi step form

@mikejw 

Pull Request for POC so you can easily see changes.

### DO NOT MERGE
Ok, I think there may be a better way, which I think is more RESTful & might solved my issue!

Instead of using `[POST] /users` to create a user & `[POST] /details` to create details but with a `user.id`.

Would it not be more RESTful to use `[POST] /users/1/details` instead? @rorywoodward