Decouple presentation layer from business logic

Right now some things are pretty closely coupled with the presentation layer (i.e., the Svelte components). A pretty egregious example of this is the [MultipleStopsDisambiguator](https://github.com/tjhorner/gtfs-janitor/blob/main/src/lib/components/MultipleStopsDisambiguator.svelte). To make everything more testable, we need to decouple the business logic and presentation layer.