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.

(cherry picked from commit 51cef01c29)

zerver/lib/reminders.py

@@ -32,7 +32,7 @@ def get_reminder_formatted_content(
     if note:
         note = normalize_note_text(note)
 
-    if message.is_stream_message():
+    if message.is_channel_message:
         # We don't need to check access here since we already have the message
         # whose access has already been checked by the caller.
         stream = Stream.objects.get(