hotfix(ScheduleDirection): fix line diagram for routes with repeated stops

[thecristen]

No ticket - fixing a bug on dev with the 217 & 120 bus route schedule pages.

Background

• refactor(RoutePattern): get stop_ids from trip ++ related improvements #1755

The recent PR updated how stops were being fetched for the line diagram, but came with corrected data that broke the line diagram code on at least two pages. It turns out the recursive code that parses the stops to draw the branches on the line diagram assumes that each stop ID is unique, which is assuredly not the case for the 217 and 120 routes, each of which repeat at least one stop along its route.

Highlighted changes

The approach I took here was to just not compute the stop tree for pages for a ferry or bus route. We don't need it - these either don't have line diagrams or have simple lines that won't branch.

Backend

• Consolidated the two places that were computing a stop tree into one function
• Adjusted that function to return different data depending on the route type - allowing us to send nil stop tree

Frontend

• Refactored many components to work with a null stop tree.
• Use the list of stops from the backend if the stop tree is null

Fixed 217 route

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General checks

• Are the changes organized into self-contained commits with descriptive and well-formatted commit messages? This is a good practice that can facilitate easier reviews.
• Testing. Do the changes include relevant passing updates to tests? This includes updating screenshots. Preferably tests are run locally to verify that there are no test failures created by these changes, before opening a PR.
• Tech debt. Have you checked for tech debt you can address in the area you're working in? This can be a good time to address small issues, or create Asana tickets for larger issues.

New endpoints, or non-trivial changes to current endpoints

• Have we load-tested any new pages or internal API endpoints that will receive significant traffic? See load testing docs
• If this change involves routes, does it work correctly with pertinent "unusual" routes such as the combined Green Line, Silver Line, Foxboro commuter rail, and single-direction bus routes like the 170?

[kotva006]

We really shouldn't be using indexes as keys, for the underlying list items could change, but the index does not. Is the id not unique here? Is there some other way to get a unique id? (Like name-id-zone)

[thecristen]

The routeStop.id is not guaranteed to be unique amongst the list of stops. Because of this edge case where stops are repeated along the route. But maybe I can add a number to each item in the list and use that as the key...

[thecristen]

@kotva006 I've pushed an update to change this.

[kotva006]

Looks good