Use websocket instead of polling on multiplayer room events.

[astro-angelfish]

We could make more use of notification websocket like chat for multiplayer room events. To avoid flooding them to users, we need to add command that allows user to subscribe to multiplayer rooms they are interested in and not private.

This could be useful for tournament data crawling with passive requests and developing event driven bots. The most important thing is that I don't have to write a loop to poll the things from the match and filter boring items.

Could be helpful for customized multiplayer bots if resolved with #11426 or any other interface provided for room manipulation instead of legacy IRC protocol.