Consolidate Board Bus logic

[Gerzer]

Currently, Board Bus application logic—specifically, the code that sets the on-bus-not-on-bus state—is spread out across two different SwiftUI views. Additionally, with the development of Board Bus with Siri and Automatic Board Bus, there will be many places in our code that need to perform the action of boarding or leaving a bus. We should consolidate this logic in a new BoardBusManager singleton actor. This has the added benefit of removing the need to compile MapState into targets that don’t really need to care about the visual map but that do work with Board Bus.

[Gerzer]

Due to the interdependent nature of the code changes, we’ll consolidate #28 into this issue.