message: Use .is_channel_message column instead of is_stream_message().

This avoids a potential unnecessary message.recipient fetch required by
is_stream_message(). is_stream_message() methods precedes the addition
of the denormalized is_channel_message column and is now unnecessary.

In practice, we usually fetch Message objects with `.recipient` already,
so I don't expect any notable performance impact here - but it's still a
useful change to make.

zerver/lib/test_classes.py

@@ -2775,6 +2775,7 @@ class PushNotificationTestCase(BouncerTestCase):
             rendered_content="This is test content",
             date_sent=timezone_now(),
             sending_client=self.sending_client,
+            is_channel_message=type == Recipient.STREAM,
         )
         message.set_topic_name("Test topic")
         message.save()