Display a button to join a ticket conference for each user

[mxsph]

Is your feature request related to a problem? Please describe.

If you join a conference to a ticket you did not create and you are not a lecturer, you can only join this conference via the conference list and not via a button on the ticket.

Describe the solution you'd like
It would be practical if the button to join a conference (if available) is displayed for each user on the ticket. We should probably consider that this button is not displayed if an invisible conference is linked.

Describe alternatives you've considered
Change nothing