[meta] Create a UI for promotions

Post-beta1 we want to ship with a fully functional discount UI, like the one commerce_discount provided in 1.x.

To account for potentially large numbers of discounts (thousand stores each with 5 discounts each, for example. Or 10 stores with 100 each), discounts need to be content entities. They need to embed and serialize conditions, ideally using the Rules API (this replaces the 7.x inline_conditions).