feat(SF2.0/UpcomingDepartures): show no realtime

[thecristen]

Scope

Asana Ticket: [SF/UD] All Modes: "No predictions" message (plus next N scheduled trips for non-subway)

Implementation

Adds even more logic to upcoming_departures/1, now outputting two new states:

• {:no_realtime, upcoming_departures} which will let us show schedule-only output
• :no_realtime with no other accompanying output

Screenshots

How to test

Find a route and stop that are in service but are without predictions. This can be tricky. Commuter rail stops are a good bet, since predictions are not usually available far in advance of the trip, and trips are spaced apart. Sometimes light rail termini will show this too.
Running the code locally you can set all_predictions = [], though it's not a 100% faithful representation since vehicle data will still be around to populate bus crowding.