Pull guest access token out of the auth session params, otherwise it will...

Pull guest access token out of the auth session params, otherwise it will break if you open the email on a different device.

synapse/rest/client/v2_alpha/register.py

@@ -152,6 +152,7 @@ class RegisterRestServlet(RestServlet):
 
         desired_username = params.get("username", None)
         new_password = params.get("password", None)
+        guest_access_token = params.get("guest_access_token", None)
 
         (user_id, token) = yield self.registration_handler.register(
             localpart=desired_username,