WebSocket communication: List online users

[Erik Michelson]

Is your feature request related to a problem? Please describe.
no

Describe the solution you'd like
The online users sidebar menu should contain the list of currently active users.

Describe alternatives you've considered
none

Additional context
none

[Erik Michelson]

GUI and redux state methods/types were implemented with 6d7bd01

Only the websocket connection for this is missing yet.

[Tilman Vatteroth]

Fixed in #2118