Add support for product-level subscriptions #20
Description
Implement support for subscriptions that are associated with products.
- Drop-down to select billing plan on product page, if said product has any associated
- Drop-down for products on cart page
- Ability to remove subscription plan from product on cart page (if product can be purchased without subscription)
- Add support for subscription incentive discount, subscription trial gift and trial period
- Discount is shown and calculated in cart
- Trial gift is added to cart as expected
- Trial period is visually communicated
- Ensure cart accounts for saving credit cards in case of subscription in checkout