feat(TripPlanner): add itinerary tags

[thecristen]

Summary of changes

Asana Ticket: Trip planner backend changes and Trip planner frontend changes

Lots of changes here!

Using the OpenTripPlannerClient

I've extracted OpenTripPlanner-touching code into a standalone open_trip_planner_client library, which I hope to move into our Github org and share with @mbta/mobile-app soon.

So far it primarily implements the plan/3 function to query OTP's plan, but under the hood it parses input arguments, dynamically constructs the GraphQL query, and handles very light parsing (mainly just error handling), while also applying tags (thanks mbta/mobile_app_backend#34 !).

This PR updates our codebase to use the client's implementation. Half of the changes in this PR involved removing various bits of our code and adjusting others.

Mocking and test data generation

I added two long-recommend libraries, Mox and ExMachina, along with Faker for generating data.

The client library exposes a behavior, OpenTripPlannerClient.Behaviour, which we use with the Mox library to provide some mocked functionality in tests.

Separately, we replace the MockPlanner functions with test factories created using ExMachina and Faker.

The other half of the bulk of changes in this PR are from refactoring many tests.

Adding itinerary tags to the UI

Each itinerary should have at most one tag, and all tagged itineraries are shown before the rest.

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[thecristen]

Tagging has been updated. This screenshot shows both (1) tagged itineraries shown before untagged and (2) tag prioritization (#2 is labelled just "least walking" instead of both "least walking" and "shortest trip")