better support in UI for subscribed calendars

Coming from:
https://help.nextcloud.com/t/can-not-export-subscribed-calendar-exporting-regular-calendar-works-fine/115862

External/subscribed calendars will behave different than regular calendars.

• when adding the whole Nextcloud calendar account (e.g. add caldav account in Apple Calendar) it is the original private link that is made available there, not the events that have been imported in Nextcloud calendar.
• export from Nextcloud calendar will not work (error: "This is the WebDAV interface")

These features would be useful in Nextcloud calendar:

• make it visible to the user what are native calendars and what are subscribed calendars (see screenshot for how Apple calendar identifies subscribed calendars)
• for a subscribed calendar: disable the option to copy private link, as this link is used for exporting and will not work and/or change the error
• for a subscribed calendar: add option to view/edit the URL of the subscribed calendar

Start with the issue description and the linked help.nextcloud.com report, then trace the calendar UI handling for native versus subscribed calendars. Done means subscribed calendars are distinguishable and provide the requested private-link and subscribed-URL behavior without the reported export confusion.