Allow unsubscribing holiday calendars from the picker modal

[ChristophWurst]

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

As a user I want to quickly unsubscribe from holiday calendars from within the picker modal. I don't want to exit the modal and use the Delete action on the calendar entry in the sidebar.

Describe the solution you'd like

If a Thunderbird holiday calendar has already been subscribed to, render a Unsubscribe button that deletes the webcal subscription.

Describe alternatives you've considered

No response

Additional context

This is part of #962 and a limitation of the first implementation of holiday calendars at #5167.

[ChristophWurst]

Ideas for the implementation: #962 (comment)