Submit nested registrant form and don't require separate submits

[JPustkuchen]

Especially if there is only one registrant / contact type existing the current complex structure is bad for UX. The separate selection of a person, entering its data (rng_contact e.g.), submitting the identity, entering further registration data and then submitting is way too complicated in a simple circumstance with only one bundle of each type. It would be very helpful to have a closer look at those simpler scenarios and their UX.

My suggestion in this case would be to NOT require to submit identity form (validateIdentityElement) prior and separate to the registration form at all. With only one registrant allowed it would be best to handle the whole form at once, at least optionally.
The best would be to hide the identity submit button in this case, but that would be bonus, I think.

The attached screenshot shows my suggestion. It already makes use of the patch from #116 which is also part of the simplification.

[JPustkuchen]

Sadly I wasn't able to found out who to handle the inner identity form in Registrants.php so that it's values are used when clicking the main "Save" button. It would be nice to get some support here. I think that may even be an optimization for all other cases where the above form is filled (for the last participant) and save one (optional) click in that case if the main form can handle all values.