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

I think you need to use curly braces when using `change` -

~~~ruby
expect { crc.save! }.to change { CampRegistration.count }.by(1)
~~~

https://www.relishapp.com/rspec/rspec-expectations/v/2-0/docs/matchers/expect-change