Add is_webhook option to authentication decorats.

Modified:
authenticated_rest_api_view
authenticated_api_view and validate_api_key.

zerver/lib/test_helpers.py

@@ -1,9 +1,11 @@
 from __future__ import absolute_import
+from contextlib import contextmanager
 from typing import cast, Any, Callable, Generator, Iterable, Tuple, Sized, Union, Optional
 
 from django.test import TestCase
 from django.template import loader
 from django.http import HttpResponse
+from django.utils.translation import ugettext as _
 
 from zerver.lib.initial_password import initial_password
 from zerver.lib.db import TimeTrackingCursor
@@ -33,6 +35,9 @@ from zerver.models import (
     UserProfile,
 )
 
+from zerver.lib.request import JsonableError
+
+
 import base64
 import os
 import re