Rename drug_order.start_date to date_activated

Based on discussion here and talk here. We need to distinguish between the date when the order is activated and the one when the order actually starts, this is important when it comes to drug orders where a doctor might prescribe a medication for a patients that they need to start taking at a later date. Therefore we need to rename orders.start_date to date_activated and it acts as the activation date where as scheduled_date in conjunction with urgency field will be used to represent the date when the patient is to start taking the medication.