Fix joining playlists room sometimes not selecting the first item

[smoogipoo]

Prereqs:

• Return populated room object when joined osu-web#11675

When the room is joined and has a non-null ID, the playlists subscreen expects the playlist to be populated to select the first item:

osu/osu.Game/Screens/OnlinePlay/Playlists/PlaylistsRoomSubScreen.cs

Lines 89 to 97 in 23ef8fd

	private void updateSetupState()
	{
	if (Room.RoomID != null)
	{
	// Set the first playlist item.
	// This is scheduled since updating the room and playlist may happen in an arbitrary order (via Room.CopyFrom()).
	Schedule(() => SelectedItem.Value = Room.Playlist.FirstOrDefault());
	}
	}

This will now be possible after the above osu!web change.

[smoogipoo]

This is a char-for-char copy of CreateRoom(). I think everything continues to work correctly without the room.CopyFrom() line, as opposed to CreateRoom() which requires it, but I hope to remove both cases of this in the future.

Just keeping it around to keep things uniform. Can remove if desired.