preserving Mobility Explorer

[drewda]

• transfer repo to https://github.com/transitland/mobility-explorer/
• set up Netlify deployment to https://mobility-explorer.netlify.com/
• update code to work on https://mobility-explorer.netlify.com/
• remove branding and copy that's no longer relevant
• bring in docs from remove Mobility Explorer docs & images valhalla/valhalla-docs#204
• use tangram.js from a source other than mapzen.com
• use tiles from a source other than mapzen.com
• switch off Mapzen Search
• temporarily disable functions that rely on Mapzen Mobility APIs
• offer alternatives to Mapzen Mobility APIs (isochrone and map-matching --> alternatives to Mapzen Mobility APIs #522

//cc @meghanhade @irees @burritojustice @rmglennon @hanbyul-here