Add support for Google OAuth2 authentication

This replaces OpenID authentication, which is going away soon, but
provides an upgrade path where we can migrate users that already have
a Google OpenID setup to the new system transparently.

config/example.application.yml

@@ -88,6 +88,10 @@ defaults: &defaults
     - ".*\\.google\\.ru/.*"
   # URL of Overpass instance to use for feature queries
   overpass_url: "//overpass-api.de/api/interpreter"
+  # External authentication credentials
+  #google_auth_id: ""
+  #google_auth_secret: ""
+  #google_openid_realm: ""
 
 development:
   <<: *defaults