UI : Add the possibilty to logout a connected client

[rollandf]

After a race is done, if the connected clients are not closed, no new client can connect.

When running a competition, it will be helpful to be able to logout connected clients to enable new competitors to log in.

Note that it should not be possible to disconnect a client when a game is running.

[nirs]

Hey @rollandf do you plan to work on this?

[rollandf]

Hi @nirs , I am not planning to work on it.