Build top-line bus and subway ridership cards in the style of `LineCard`

[idreyn]

One commonly-requested feature is to break the total ridership into "Total Subway Ridership" and "Total Bus Ridership" charts (as usual the SL makes a total mockery of this dichotomy). I think it would make the most sense to do this in the style of LineCard, i.e. this component:

This task would have two parts:

• On the Python side, writing new data to aggregate "total bus" and "total subway" fields in data.json. We've already done this to get the TopLine sparklines, so we're almost there!
• On the TypeScript side, adapting or generalizing LineCard to show this data with appropriate styling.