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.
(cherry picked from commit 51cef01c29)
8c9c39059d