Show slowest routes on homepage

[hathix]

Along with #585, this is a feature that SFTR/SFMTA would be interested in. They want to find the "hotspots" (segments, or adjacent stop-stop pairs, that are the slowest) in the muni system. Bubbling this information up on the dashboard would make it easy to find hotspots.

Perhaps add an optional column to the routes table (as part of #553) that lets users see the slowest routes in each route.

Or show colors for each segment like Google Maps does so users can see a heatmap of the best/worst areas.