Improve web app visualization

[dabreegster]

• [ ] Plumb along and display zones for origins/destinations too?
• [ ] Scale O/D circles based on count, and include histograms for them too?
• [ ] Try rank/ordering for line width (1st most important edge, 2nd...)
• [ ] Auto-set the max for line width value, by plumbing along the max count in the output metadata

[dabreegster]

The pmtiles simplification right now is pretty broken (and slow to generate!).

-j '{"*": ["any", [">=", "$zoom", 13], [">", "count", 10]]}'

This only shows roads with >= count 10 at low zooms, but shows everything beyond 13. Problem is, these have to be tuned for each example...