Fixed #27518 -- Prevented possibie password reset token leak via HTTP Referer header.

Thanks Florian Apolloner for contributing to this patch and
Collin Anderson, Markus Holtermann, and Tim Graham for review.

tests/auth_tests/client.py

@@ -0,0 +1,41 @@
+import re
+
+from django.contrib.auth.views import (
+    INTERNAL_RESET_SESSION_TOKEN, INTERNAL_RESET_URL_TOKEN,
+)
+from django.test import Client
+
+
+def extract_token_from_url(url):
+    token_search = re.search(r'/reset/.*/(.+?)/', url)
+    if token_search:
+        return token_search.group(1)
+
+
+class PasswordResetConfirmClient(Client):
+    """
+    This client eases testing the password reset flow by emulating the
+    PasswordResetConfirmView's redirect and saving of the reset token in the
+    user's session. This request puts 'my-token' in the session and redirects
+    to '/reset/bla/set-password/':
+
+    >>> client = PasswordResetConfirmClient()
+    >>> client.get('/reset/bla/my-token/')
+    """
+    def _get_password_reset_confirm_redirect_url(self, url):
+        token = extract_token_from_url(url)
+        if not token:
+            return url
+        # Add the token to the session
+        session = self.session
+        session[INTERNAL_RESET_SESSION_TOKEN] = token
+        session.save()
+        return url.replace(token, INTERNAL_RESET_URL_TOKEN)
+
+    def get(self, path, *args, **kwargs):
+        redirect_url = self._get_password_reset_confirm_redirect_url(path)
+        return super(PasswordResetConfirmClient, self).get(redirect_url, *args, **kwargs)
+
+    def post(self, path, *args, **kwargs):
+        redirect_url = self._get_password_reset_confirm_redirect_url(path)
+        return super(PasswordResetConfirmClient, self).post(redirect_url, *args, **kwargs)