feat(TripPlanner.Form): use Ecto.Schema to create a form

[thecristen]

Summary of changes

Asana Ticket: a part of Trip planner form Phoenix.LiveComponent

It's my first foray using Ecto, and really my first time in-depth with working with forms in Phoenix in general, but I've been working on a form with this schema as a base! This only includes the from and to locations for now, and the leave/arrive choice, mode selection, and wheelchair: true choices will be added later. Open to feedback on how I might improve this.

• For each location (origin/destination) I wanted to be able to capture the coordinates, name, and stop ID (if a stop is chosen), since these are what get passed into OTP.
• I had a lot of trouble getting the validation to do what I wanted, so I ended up writing a custom one that adds an error to the location field if any of those sub-fields were invalid/missing. But I'd really rather have been able to make cast_embed(:from, required: true, required_message: "Please select an origin") or its invalid_message counterpart work.

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