alternatives to Mapzen Mobility APIs

[drewda]

Mobility Explorer has depended upon Mapzen Mobility APIs for isochrone accessibility analysis and for map-matching of GPX files.

Mapzen services are shutting down, so we're temporarily disabling this functionality.

In the future we'll try to set up Mobility Explorer to allow users to choose to use:

• other hosted providers of Valhalla-based APIs
• Valhalla running on your own Mac using homebrew-valhalla
• Valhalla running on your own computer using valhalla/docker

//cc @irees @meghanhade @burritojustice ...