Product categories can now be marked as "cross-selling categories", causing them to
appear in the add-on checkout step as additional recommendations, depending on
their cross-selling visibility (always, only if certain products are already in the cart, or
only if they qualify for a discount according to discount rules).
---------
Co-authored-by: Raphael Michel <michel@rami.io>
Co-authored-by: Richard Schreiber <schreiber@rami.io>
