Add, remove & view promo codes for memberships and on invoices

New features:

1. Allow staff to add/remove promo codes from active subscriptions.

2. When viewing a subscription on a user's profile, show when promo code has been applied and the details of the promo code.

3. Add promo code discounts as line items on invoices.

4. On the Payments tab on user profiles, show upcoming scheduled invoices with the final price (after discounting), so it's possible to see what the user will pay in future and when the promo code discount will run out.

5. Simplify the dates to show only the following, ensuring that both are in local time

   1. Invoice Date

   2. Payment Date