Responders grid should automatically check for new requests

[azasypkin]

Summary

Today, the user must repeatedly collapse and expand the responder requests panel to get the latest captured requests, which is very inconvenient. The responder grid should automatically and periodically check for new requests for all "expanded" responders instead.

The proper way to do this would be SSE, but Actix Web doesn't have native SSE support yet and one needs to track clients manually, which is error-prone. Even though using WebSocket for this use case will have some overhead, the actix_web_actors is much more mature and we'll likely need to use WebSockets elsewhere in the future anyway.

/cc @legrego