Allow consecutive upsells (make downsells optional)#2781

Funnel steps are forced to alternate upsell/downsell, so we can’t place two upsells in a row. If we delete a downsell, the next upsell turns into a downsell, and buyers who accepted the previous upsell never see it.

Please let us set each step’s type manually, so we can build flows like Upsell → Upsell → Upsell.