Enable a parent to delete a camp_registration

WIP
* Delete functionality works but I've slightly broken the new camp_registration functionality - in that error messages for invalid forms do not show.
* camp_registrations_controller#create needs work to re-render the form with persisted form values.
* 3 failing tests in camp_registration_collection_spec

Better to say

~~~ruby
@camp_registrations = current_parent.camp_registrations
~~~