Use globally-available reCAPTCHA source

As google.com over HTTPS is not available in China and other countries
with large blocks, use reCAPTCHA from recaptcha.net per this FAQ:

https://developers.google.com/recaptcha/docs/faq#can-i-use-recaptcha-globally

app/assets/javascripts/vue_shared/components/recaptcha_modal.vue

@@ -19,7 +19,7 @@ export default {
   data() {
     return {
       script: {},
-      scriptSrc: 'https://www.google.com/recaptcha/api.js',
+      scriptSrc: 'https://www.recaptcha.net/recaptcha/api.js',
     };
   },