Multi-class races

Let's add a field to the race setup section for "Opponents". By default we'll have "same class". Let's then define a dedicated dataset: a list of multiclass configurations. Each multiclass configuration is a set of classes. Any class may only be part of a single multiclass configuration. Whenever the race is in a class that's part of a multiclass configuration, we'll show the player on the race setup section which classes to set opponents to.

Alternatively, we could define multiclass configurations as:

* A set of (discipline, class) tuples. This allows a class that exists in multiple categories (say, GT3) to have separate multiclass configurations per discipline, at the cost of having to duplicate the entries for each discipline if we do not want them to be separate
* A set of (discipline, rank, class) tuples. This allows even more granular control, at the cost of even more duplication for cases where we do not want that control.