[CALENDAR] Alert if an appointment does not take place

CrocoClick adds a smart alert that notifies the assigned user when an appointment is created without a meeting place or link (Zoom/Meet/Teams integration failing, empty field or manual addition). The alert can be sent by email and/or in-app notification according to each member's preferences, can be set in the calendar's notification settings, and helps avoid no-shows by enabling the event to be quickly corrected before the big day.