Replace v2 Google Maps with a more flexible leaflet-based mapping system

New options in secrets.yml allow selection of a mapping provider and
tile set from Stamen, MapBox, Google, and others.

This was kicked off by #55, but goes a bit beyond addressing that issue.
[!] Deprecate config/geocoder_api_keys.yml in favor of secrets.yml
Add note about config/geocoder_api_keys.yml to change log
Add appropriately-green map marker
Fix precompiling of leaflet_google_layer.js
Alias MappingHelper#map to #google_map for compatibility
Add a note about mapping js includes for theme authors
Add note about #google_map => #map div renaming