Allow saving orders with a future scheduled date

The current validations in Order entry uses start_date(date_activated) and auto_expire_date to check if drugs orders overlap. To support drug orders in future date it should also consider scheduled_date and urgency.

Notes from : http://notes.openmrs.org/Design-Forum-2014-07-28

Summary: we do not allow overlapping time frames, defined by
start of period = drugOrder.urgency == SCHEDULED ? drugOrder.dateScheduled : drugOrder.dateActivated
end of period = drugOrder.dateStopped ? drugOrder.dateStopped : drugOrder.autoExpireDate (where null means "it goes forever")

An order is active from date_activated through (date_stopped OR auto_expire_date)
An order is started as of scheduled_date OR date_activated

Proposal for using scheduled date:
https://talk.openmrs.org/t/proposal-on-how-to-manage-start-date-for-orders/443
Described on Order Uniqueness Constraints:
https://wiki.openmrs.org/x/GYCMAw