CalDav multiple agenda support

[vanilla-extracts]

Hi! Currently, the nextmeeting CalDav implementation supports only one calendar, by supplying it with caldav_url.

Other systems construct the endpoint, let me explain a bit If your calendars are stored in this endpoint https://agenda.example.com/dav/calendars/ you can construct various endpoints with the user and the calendar so https://agenda.example.com/dav/calendars/$USER/$CALENDAR.

If we make caldav_calendar a list we could construct multiple agenda endpoints and thus support multiple calendars from the same user.