Adds buttons to test availability of server from public internet

frontend/js/app/api.js

@@ -685,6 +685,16 @@ module.exports = {
                 return fetch('post', 'nginx/certificates/' + id + '/renew', undefined, {timeout});
             },
 
+            /**
+             * @param  {Number}  id
+             * @returns {Promise}
+             */
+            testHttpChallenge: function (domains) {
+                return fetch('get', 'nginx/certificates/test-http?' + new URLSearchParams({
+                    domains: JSON.stringify(domains),
+                }));
+            },
+
             /**
              * @param   {Number}  id
              * @returns {Promise}