Added "places" pages to show maps of where members are

https://www.pivotaltracker.com/story/show/53848631

Notes:
 - Replaced Google Maps with OpenStreetMap for licensing reasons.
 - Replaced Geocoder queries with direct calls to Nominatim (OSM's lat/lon lookup service) because Nominatim-via-Geocoder was strangely slow (https://github.com/alexreisner/geocoder/issues/504).
 - There are still a couple of outstanding JavaScript/CSS tweaks we need (see PT), but we thought it was worth a pull request for people to comment on.