Add an explicit interface for calls from outside the `views` package

Currently it's not immediately clear what other parts of the system depend on views.
Adding an explicit interface should
- simplify changes behind the interface
- simplify things for consumers, because right now they can reuse individual parts (like ViewsService) and then have to deal with our data structures themselves. => Repeated logic, subtle bugs (because the intricacies of the views system might not be intuitive for everyone), and very tight coupling.

If the called functionality is generic, the interface could be defined in views. If not, it might make more sense to define multiple interfaces on the side of consumers and make views implement them