Make recaptcha optional on signup page.

config/danbooru_default_config.rb

@@ -619,6 +619,18 @@ module Danbooru
 
     def aws_sqs_cropper_url
     end
+
+    # Use a recaptcha on the signup page to protect against spambots creating new accounts.
+    # https://developers.google.com/recaptcha/intro
+    def enable_recaptcha?
+      Rails.env.production? && Danbooru.config.recaptcha_site_key.present? && Danbooru.config.recaptcha_secret_key.present?
+    end
+
+    def recaptcha_site_key
+    end
+
+    def recaptcha_secret_key
+    end
   end
 
   class EnvironmentConfiguration