Appointments: Allow conflict checking with shared read-only calendars (currently limited to writable calendars)

[kesselb]

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

I'm unable to pick a shared read-only calendar in the appointment config to check for conflicts.

Describe the solution you'd like

Let me select the read-only calendar.

Describe alternatives you've considered

Copy the events to my personal calendar.

Additional context

No response