Get a specific room info

[BECATRUE]

Feature you want to implement

I will implement to get the room info for an user in a room.

It sends to all clients in real-time through a web socket of ~/room/{room_id}/info/.

Now, it will contain only players info. Concretely, the return structure is as follows:

{
 "players": ["seokhwan", "jaehun"]
}

How the feature is implemented

The implementation is similar to #7.