FEATURE: log to STDOUT using Rails 5 env var (#18880)

Rails introduced a environment variables RAILS_LOG_TO_STDOUT in the
template for new projects here: https://github.com/rails/rails/pull/23734

This commit adds the same code to the discourse production environment,
making it easy to configure logging to stdout in production for
environments which already support log collection via stdout.

config/environments/production.rb

@@ -67,4 +67,8 @@ Discourse::Application.configure do
   end
 
   config.active_record.dump_schema_after_migration = false
+
+  if ENV["RAILS_LOG_TO_STDOUT"].present?
+    config.logger = ActiveSupport::TaggedLogging.new(Logger.new(STDOUT))
+  end
 end