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

Take a look at more specific validation specs using `errors_on`

https://www.relishapp.com/rspec/rspec-rails/v/2-3/docs/model-specs/errors-on