Pass new read_by_sender flag when sending messages

[timabbott]

This will make it explicit to 8.0+ servers that the newly-sent message should be marked as read by our user from the outset. The server already determines this through a hack where it inspects the User-Agent header, but it'd be good to make that hack unnecessary to help with eventually removing it on the server.

See https://zulip.com/api/send-message#parameter-read_by_sender and https://chat.zulip.org/#narrow/stream/378-api-design/topic/read_by_sender.20flag.20on.20send-message/near/1702407 for details.