Specify Google Analytics ID in secrets.yml
Created by: pmather
Like many other configuration settings, we now set the config.google_analytics_id setting if there exists a "google_analytics_id" setting in config/secrets.yml for the current environment.
If a key is not set in config/secrets.yml, Rails.application.secrets will return nil when attempting to reference that key. By testing for nil we attempt to mimic the original behaviour whereby config.google_analytics_id defaults to being unset in the code.