Map view: toggle rides starting vs ending at a given station

[ryan-williams]

Per reddit comment, i currently surface where each station's originating rides end up (example), but the reverse could be interesting.

On a monthly timescale (as it's currently set up), I expect station pairs to have similar counts in each direction, so not sure showing the other direction will illuminate much, but it's worth trying, and in the future it would be cool to visualize that kind of thing over the course of a day, etc.