Allow multiple product lists for shifts

[JobDoesburg]

Is your feature request related to a problem? Please describe.

Among others because of #1654, it would be nice to be able to sell both pizzas and regular borrel products in a single shift

Describe the solution you'd like

Let product_list be a ManyToMany field, and let each shift construct its own effective product list consisting of the union of all product lists that are selected

Motivation

So both pizzas and beer can be sold in the same shift

Describe alternatives you've considered

Creating product lists like 'borrel with pizza' and 'borrel without pizza'. Which works absolutely fine! So priority is maybe_someday

[se-bastiaan]

Doesn't this depend on the solution we choose for the food events?

[JobDoesburg]

Doesn't this depend on the solution we choose for the food events?

Yes, but even without food events at all it could be a useful feature