Total-based offer conditions

Right now, the upsell offers only appear when the cart contains certain items, but we want to also allow offers that are based on the total value of the cart.

This opens up for offers like “10% for purchases above 50€”, and can help drive more sales.

Unlike item-based offers, these offers should be shown before they are eligible, with a progress bar towards the next “level”.

Since discounts needs to be applied to items in the cart, discounts from an offer like this will likely be applied to each item individually.