Improve WebSocket UI

After v7 there are a couple of things that can be improved, I don't want them to be forgotten:

1. Whenever a new WebSocket message arrives the list jumps back to the top. That means you cannot read through all messages by scrolling down while the socket is open because it will keep jumping back (ttps://github.com/mitmproxy/mitmproxy/pull/4486#issuecomment-796578909)
2. Opcode.TEXT message should not be passed through bytes_to_escaped_str but render as the original string (new text property) (https://github.com/mitmproxy/mitmproxy/pull/4650#issuecomment-868676866)