display expired price set fields for backend regstration

currently, when you register someone via the backoffice, only date-valid price fields are displayed. expired options are hidden from display. but for staff using the backend, it's often necessary to have access to the full set of price fields. 

in CRM-16456 an exception was made so that when editing an existing registration the full set of options were made available to the backoffice form. I believe that should be extended to also be available when creating a new registration.

because this does involve a change in functionality, I think it deserves some discussion. I could see some resistance for orgs that are heavy users of date restricted fields and want to strictly prevent staff from mistakenly selecting what is now an expired option. but even those cases, I would think they would at times need the flexibility this change would provide.