switch from google.com to recaptcha.net for reCAPTCHA (#4731)

* add trivial clarification about jemalloc

* switch from google.com to recaptcha.net

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

README.rst

@@ -199,6 +199,8 @@ by installing the ``libjemalloc1`` package and adding this line to
 
     LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.1
 
+This can make a significant difference on Python 2.7 - it's unclear how
+much of an improvement it provides on Python 3.x.
 
 Upgrading an existing Synapse
 =============================

synapse/config/captcha.py

@@ -47,5 +47,5 @@ class CaptchaConfig(Config):
         #captcha_bypass_secret: "YOUR_SECRET_HERE"
 
         # The API endpoint to use for verifying m.login.recaptcha responses.
-        recaptcha_siteverify_api: "https://www.google.com/recaptcha/api/siteverify"
+        recaptcha_siteverify_api: "https://www.recaptcha.net/recaptcha/api/siteverify"
         """

synapse/handlers/register.py

@@ -460,7 +460,7 @@ class RegistrationHandler(BaseHandler):
         lines = response.split('\n')
         json = {
             "valid": lines[0] == 'true',
-            "error_url": "http://www.google.com/recaptcha/api/challenge?" +
+            "error_url": "http://www.recaptcha.net/recaptcha/api/challenge?" +
                          "error=%s" % lines[1]
         }
         defer.returnValue(json)
@@ -471,7 +471,7 @@ class RegistrationHandler(BaseHandler):
         Used only by c/s api v1
         """
         data = yield self.captcha_client.post_urlencoded_get_raw(
-            "http://www.google.com:80/recaptcha/api/verify",
+            "http://www.recaptcha.net:80/recaptcha/api/verify",
             args={
                 'privatekey': private_key,
                 'remoteip': ip_addr,

synapse/rest/client/v2_alpha/auth.py

@@ -33,7 +33,7 @@ RECAPTCHA_TEMPLATE = """
 <title>Authentication</title>
 <meta name='viewport' content='width=device-width, initial-scale=1,
     user-scalable=no, minimum-scale=1.0, maximum-scale=1.0'>
-<script src="https://www.google.com/recaptcha/api.js"
+<script src="https://www.recaptcha.net/recaptcha/api.js"
     async defer></script>
 <script src="//code.jquery.com/jquery-1.11.2.min.js"></script>
 <link rel="stylesheet" href="/_matrix/static/client/register/style.css">

synapse/static/client/register/index.html

@@ -4,7 +4,7 @@
 <meta name='viewport' content='width=device-width, initial-scale=1, user-scalable=no, minimum-scale=1.0, maximum-scale=1.0'> 
 <link rel="stylesheet" href="style.css">
 <script src="js/jquery-2.1.3.min.js"></script>
-<script src="https://www.google.com/recaptcha/api/js/recaptcha_ajax.js"></script>
+<script src="https://www.recaptcha.net/recaptcha/api/js/recaptcha_ajax.js"></script>
 <script src="register_config.js"></script>
 <script src="js/register.js"></script>
 </head>