MM-56071: Thread presence indicator (#25694)

We also track the channelID of the thread opened.

Additionally on every connection create or re-connect
with an existing queue, we reset the active state to empty
to avoid any edge-cases.

https://mattermost.atlassian.net/browse/MM-56071

```release-note
NONE
```

server/public/model/websocket_client.go

@@ -342,6 +342,14 @@ func (wsc *WebSocketClient) UpdateActiveTeam(teamID string) {
 	wsc.SendMessage(string(WebsocketPresenceIndicator), data)
 }
 
+// UpdateActiveThread sets the channel id of the current thread that the user is in.
+func (wsc *WebSocketClient) UpdateActiveThread(channelID string) {
+	data := map[string]any{
+		"thread_channel_id": channelID,
+	}
+	wsc.SendMessage(string(WebsocketPresenceIndicator), data)
+}
+
 func (wsc *WebSocketClient) configurePingHandling() {
 	wsc.Conn.SetPingHandler(wsc.pingHandler)
 	wsc.pingTimeoutTimer = time.NewTimer(time.Second * (60 + PingTimeoutBufferSeconds))