feat: add shuttle route definition to shuttles

[meagharty]

Summary of changes

Asana Ticket: 🏹 Implement "Create/Edit Shuttle Route Definition" Basic View + Route Definition Section

The main bit to review in this PR is the shuttle form page on /shuttles/new, /shuttles/1/edit

You can populate your local database with example data for relations like Gtfs.Route, Shuttle.Shape with:
mix run priv/repo/seeds.exs
If you run into any issues with that, you may already have conflicting data in your local database; for fastest resolution, I'd recommend commenting out the conflicting parts of that script or completely resetting the data in your database via mix db.reset and re-running mix run priv/repo/seeds.exs

Reviewer Checklist

• Meets ticket's acceptance criteria
• Any new or changed functions have typespecs
• Tests were added for any new functionality (don't just rely on Codecov)
• This branch was deployed to the staging environment and is currently running with no unexpected increase in warnings, and no errors or crashes.

[meagharty]

@Whoops this was discussed in standup but since you've been out sick: this PR isn't expected to match the mockups exactly. I wanted to get out some Ecto updates and changes to shuttles.ex so we can be aligned for the other tickets encompassing this page.

I expect that we will need to make changes to the heex files; this isn't intended to set any of that in stone.

[Whoops]

Looks good! One very minor, non-blocking annoyance I noted in testing, when creating a new shuttle, perhaps the status should be defaulted to draft, so that if the user doesn't explicitly select a status they can still save it?