Don't display PARTSTAT buttons when an event has STATUS=CANCELLED

[miaulalala]

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

Currently, cancelled events still have the "accept", "decline" and "tenative" buttons shown when an event has been cancelled by the ORGANIZER.

This event was cancelled, but the modal still shows actions are possible:

Describe the solution you'd like

For a STATUS=CANCELLED for the event, the modal display is changed to clearly indicate such a status. The "accept", "decline" and "tenative" buttons are not shown any more. Instead we could display a "delete event" button.

Describe alternatives you've considered

none

Additional context

No response