Allow :site_name to be picked up from ENV vars

This mimics what we did in 6377736f4. This way provisioning an instance
from scratch doesn't require configuring things from the backoffice,
which will simplify the roll out of
https://github.com/openfoodfoundation/ofn-install/pull/734.

config/initializers/spree.rb

@@ -19,6 +19,7 @@ end
 
 Spree.config do |config|
   config.site_url = ENV['SITE_URL'] if ENV['SITE_URL']
+  config.site_name = ENV['SITE_NAME'] if ENV['SITE_NAME']
   config.shipping_instructions = true
   config.address_requires_state = true
   config.admin_interface_logo = '/default_images/ofn-logo.png'