Fixed #22894 -- admin: make add related behaviour customizable

Decouple behaviour and style from the RelatedFieldWidgetWrapper
widget so it's easier to customize.
This makes life easier for people that want to say use a modal
or just style it differently.

While at it in change_form.html start consolidating document ready
event listeners in one place.

Fixes #22894
I think it would be better to give this a more specific name like "admin_change_form_document_ready" -- does that seem okay?