Fixes #582 - adds Google Maps API key

Fixes #582 - adds the ability to set a Google Maps api key when loading
the Google Maps APIs. If no API key is set the maps APIs load
anonymously from google.
Code edits

- Makes return value from environmentVariables module consistently
undefined if an envvar doesn’t have a value. There were circumstances
where it could be undefined or an empty string.
- Updates GOOGLE_MAPS_API_TOKEN to GOOGLE_MAPS_API_KEY
- Adds haml helper for environment variables embedded in JSON in the
HTML.
Code edit

Cleans up extra space.
Code edit
Code edit
Merge pull request #620 from codeforamerica/582-maps-api-key

Fixes #582 - adds Google Maps API key