Add timer control for coupon activation and expiry

Given the admin is logged into the Saclàb backend
And the admin navigates to the Marketing => Coupon section

Then beside adding the Published and expiry date, admin can also add the start and expire TIME for the coupon

Scenario: Ensure backward compatibility for existing coupons
Given existing coupons have only date values without time
When the new timer feature is introduced
Then their time defaults should be 00:00 for published and 23:59 for expiry