Remove confusing show_descendants configuration. Fixes #3212
Don't ignore promo action beyond the first created

Currently Spree allow users to associate multiple actions, e.g
CreateAdjustment, to a promotion however it doesn't allow a promotion to
have multiple adjustments in the same other as the backend UI suggests.

It changes the `order.promotion_credit_exists?` api so that multiple
adjustments can be created for the same promotion even though only one
of them, the one with best amount, will be eligible.

Fixes #3262
Fixes #3276