Take into account when buses are running

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The front end (slightly handles this)[templates/index.html#L48], but it should be implemented in both the backend and the frontend.

The backend API should always recommend walking when the buses are not running.

In my opinion, the front end should just say something like "Sorry, the busses are not running. You'll have to walk." when the buses are not running. It should not show the start/end bus stop options.