Accept either old or new login requests during transition. (#1785)

* Accept either old or new login requests during transition.

* Made suggested edit

* Update api/login_api.py

Co-authored-by: Kyle Ju <kyleju@google.com>

api/login_api.py

@@ -28,8 +28,10 @@ class LoginAPI(basehandlers.APIHandler):
   """Create a session using the credential generated by Google Sign-In."""
 
   def do_post(self):
-    token = self.get_param('credential')
-    message = "Unable to Authenticate"
+    # TODO(jrobbins): Remove id_token after next deployment.
+    token = (self.get_param('id_token', required=False) or
+             self.get_param('credential'))
+    message = "Unable to Authenticate. Please sign in again."
 
     try:
       idinfo = id_token.verify_oauth2_token(