* calculate regular price for line item and return in API `line_subtotal`
* restore `line_subtotal` api field to previous behaviour
* correctly handle discounts on cart line items:
- return product `prices` in cart items endpoint
- calculate full price and discount on client using product
regular_price * quantity in cart
* add product prices to preview cart API data
* show product sale price discount in "save" badge:
- previously we were displaying any effective discount, e.g. from coupons which apply to whole cart
- now this badge will only display discount due to product on sale - much simpler
* rename sale badge class (discount => sale)
* clarify docs for line_subtotal - includes sale prices, not coupons
* clarify line_total docs
* add prices to schema (fix unit test)
* fix schema unit test - return `prices` as object, consistent with `totals`
* Further line total rewording
Co-authored-by: Mike Jolley <mike.jolley@me.com>
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