Discounts for WooCommerce Subscriptions 3.0.0

* New: Adding Discounts capabilities to Coupons
* New: Editing subscription cost starting from 1st payment
* Fix: Cart totals when taxes included
* Fix: Values calculated in the discount table on the product page
* Fix: Set discounted price as sale price for first payment

* New: Preventing subscription payment method switching to manual when applying 100% discount
* Fix: Few more errors when the product with a discount was permanently removed