CRM-20637 extend expired price fields in backend
Place to agree correct approach per discussions on https://github.com/civicrm/civicrm-core/pull/10423 and https://github.com/civicrm/civicrm-core/pull/10804
Place to agree correct approach per discussions on https://github.com/civicrm/civicrm-core/pull/10423 and https://github.com/civicrm/civicrm-core/pull/10804