[CHORE] Abstract sort order handling

[PaulKlauser]

Currently there are various ViewModels handling sort order, leading to duplicate code, and leaking that implementation farther up than it needs to go. We should abstract sort order so that it's not a concept filtered past the repository. It doesn't need to be part of our UI models, since the UI can just receive an already sorted list. This will also allow us to test sort ordering logic more efficiently.

Dependent on #416

[PaulKlauser]

Implementation was moved into CategoriesUseCase here: #510

However, this displaying correctly in the UI is dependent on #416. So this issue is going to be moved back to To Do, until #416 is completed.