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/channels/app/platform/websocket_router.go

@@ -82,6 +82,10 @@ func (wr *WebSocketRouter) ServeWebSocket(conn *WebConn, r *model.WebSocketReque
 			// Set active team
 			conn.SetActiveTeamID(teamID)
 		}
+		if thChannelID, ok := r.Data["thread_channel_id"].(string); ok {
+			// Set the channelID of the active thread.
+			conn.SetActiveThreadChannelID(thChannelID)
+		}
 
 		resp := model.NewWebSocketResponse(model.StatusOk, r.Seq, nil)
 		hub := conn.Platform.GetHubForUserId(conn.UserId)