Replace Google reCAPTCHA API's domain (#507)

This makes it accessible in mainland China.

Sources:
google/recaptcha#227
https://developers.google.com/recaptcha/docs/faq#can-i-use-recaptcha-globally
https://meta.stackoverflow.com/questions/402339/please-change-the-recaptcha-link-to-ensure-the-site-is-accessible-in-china

Co-Authored-By: Fredrick Brennan <copypaste@kittens.ph>

templates/header.html

@@ -17,7 +17,7 @@
 			{% for javascript in config.additional_javascript %}<script type="text/javascript" src="{{ config.additional_javascript_url }}{{ javascript }}"></script>{% endfor %}
 			{% endif %}
 		{% endif %}
-		{% if config.recaptcha %}<script src="//www.google.com/recaptcha/api.js"></script>
+		{% if config.recaptcha %}<script src="//www.recaptcha.net/recaptcha/api.js"></script>
 		<style type="text/css">{% verbatim %}
 			#recaptcha_area {
 				float: none !important;