Design LTN tool improvments: Bus Routes

## Current behaviour:

When placing modal filters on, the tool will warn the user if the filters affect existing bus routes and adjust the edits if required (e.g. substituting a modal filter for a bus gate).

When adjusting one-way streets or turn restrictions, no equivalent warning is given if the edit affects extant bus routes.

## Example

https://github.com/a-b-street/abstreet/assets/5346065/b981b4c7-45a9-46e4-ae32-634686628cb5

Video shows current behaviour whilst editing Roosevelt Drive in Oxford

* Add a modal filter - a warning is given about the effect on a bus route.
* Add one-way restrictions that make it impossible to drive along Roosevelt Drive (no warning given)
* Add turn restrictions (no straight ahead) that make it impossible to drive along Roosevelt Drive (no warning given)

## Suggested improvement:

When editing one-way streets and turn restrictions within an LTN, the user should be warned if their edits will affect extant bus routes.