channel chat: Set first loaded message ID when sending a message (#10034)

Discovered while looking into #10024.

When clicking on a reply message text, the original message should be
highlighted accordingly. However this would not work when the channel
was just created and the user is the only one that sent messages.
 
Release Notes:

- Fixed highlighting of messages when clicking on the reply message text
in the chat and there were no other messages from other users

crates/channel/src/channel_chat.rs

@@ -222,6 +222,9 @@ impl ChannelChat {
             let message = ChannelMessage::from_proto(response, &user_store, &mut cx).await?;
             this.update(&mut cx, |this, cx| {
                 this.insert_messages(SumTree::from_item(message, &()), cx);
+                if this.first_loaded_message_id.is_none() {
+                    this.first_loaded_message_id = Some(id);
+                }
             })?;
             Ok(id)
         }))