Quick draft of exporting shapesfiles for geom2gtfs

This exports a zip file of a map, containing an ESRI shapefile and JSON metadata, for easy use in [geom2gtfs](https://github.com/conveyal/geom2gtfs). It also adds an export button at the bottom on the MapSidebar to get this file.

This same approach can be used for KML export.

Todo:
- [ ] Figure out why geom2gtfs doens't produce trips
- [ ] Dynamically generate JSON instead of hardcoding
- [ ] Clean up ruby code
- [ ] Figure out where export button should live.