Trunk 4446: Support deriving auto_expire_date from duration and duration units for Drug Orders

Changes for ticket https://issues.openmrs.org/browse/TRUNK-4446
I'll change it to use uuid of concept source. 

Would you want preconditions for these as well? If needed, I guess it should look based on `where code = 'S'` here, not based on uuid of these reference terms right?