Allow read-only calendars as appointment conflict calendar

[schiessle]

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

Additionally to my regular calendar I subscribed to a calendar with public holidays. On public holidays I don't work, so people shouldn't be able to schedule a appointment on these days.

At the moment I'm not allowed to chose this calendar as a calendar to check for additional conflicts.

Describe the solution you'd like

Allow the user to add every calendar they have access to as additional conflict calendar. This way I could add the calendar with the public holidays.

I don't see a reason why we should limit the kind of calendars a user can add as conflict calendar

Describe alternatives you've considered

Importing the public holidays every year into a regular calendar and use this calendar than as a conflict calendar. But this would be extra work I have to do regularly and there is a high chance that I will miss it or that people book already appointments for the next year before I imported the new public holidays.

It is way easier to just subscribe to a calendar which updates itself.

Additional context

No response

[ChristophWurst]

Duplicate of #3786