fix(StopEvent): assign date when including schedule

[runkelcorey]

Summary of changes

Asana Ticket: 🐞 Unhandled stop_events query

1. Fixes typo in README
2. Assigns start_date as the schedule date when including schedules in stop_events queries. Not assigning the date caused this bug
3. Updates ScheduleView to format dates using a date field
4. Exposes service_id as attribute of schedules; this enables (2) and also seems useful for consumers (although I guess it hadn't been needed thus far)

How were these changes validated?

1. Updates StopEventControllerTest to check that the service ids returned by each schedule match the expected service ids

What questions should reviewers consider?

1. Are there edge cases where returning the service_id could mislead consumers?
2. Is there a cleaner way to assign the date? I feel like date_seconds and date represent just enough difference that a bigger refactor would be needed but idk

[jzimbel-mbta]

I think this looks good overall, but (as you also pointed out in the PR description) I wonder if there's a different way to associate a stop event date with a schedule than Map.put-ing it right on the struct.

[runkelcorey]

I think this looks good overall, but (as you also pointed out in the PR description) I wonder if there's a different way to associate a stop event date with a schedule than Map.put-ing it right on the struct.

Agreed. I moved the logic to associate dates and schedules out of Schedule and into StopEvent. That's also unconventional but respects the fact that stop events from 2 dates may occur