message: Respect first_visible_message_id when computing unreads.

This prevents unread counts for messages which the web UI does not
display, and does not let you mark as unread via "mark all as read."

zerver/lib/message.py

@@ -579,10 +579,11 @@ def get_raw_unread_data(
     user_profile: UserProfile, message_ids: Optional[List[int]] = None
 ) -> RawUnreadMessagesResult:
     excluded_recipient_ids = get_inactive_recipient_ids(user_profile)
-
+    first_visible_message_id = get_first_visible_message_id(user_profile.realm)
     user_msgs = (
         UserMessage.objects.filter(
             user_profile=user_profile,
+            message_id__gte=first_visible_message_id,
         )
         .exclude(
             message__recipient_id__in=excluded_recipient_ids,