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/message_report.py

@@ -59,7 +59,7 @@ def send_message_report(
             last_user_mention=last_user_mention,
         )
     else:
-        assert reported_message.is_stream_message() is True
+        assert reported_message.is_channel_message is True
         topic_name = reported_message.topic_name()
         channel_id = reported_message.recipient.type_id
         channel_name = reported_message.recipient.label()