Slot interval being removed crashes customer's bookings

[ikusteu]

Steps to reproduce:

• create a slot with two intervals (e.g. "08:00-09:00" and "08:00-09:30")
• create a customer and book an interval (e.g. "08:00-09:00")
• edit the slot to remove the interval the customer had booked ("08:00-09:00" in this case)
• bug1:
  • go to customer's bookings page and click "Calendar"
  • the page crashes: the calendar wants to show the booking, but can't find the same interval in the respective slot (as it's been deleted)
• bug2:
  • go to the attendance page and navigate to the slot
  • the customer's attendance shows the deleted interval (it shouldn't)

I don't have a direct solution. We can either:

• block deletion of already booked intervals
• add a data trigger to remove the bookings of deleted interval
• add additional (complex) data flow where the admin is prompted to confirm the deletion of the interval if it's already booked and customers are notified that the interval has been deleted

All of the aforementioned solutions open a respective can of worms...

[ikusteu]

Thinking about this, maybe we could simply not display the mismatched slots, have a cron job -> scheduled cloud function run once a day (at midnight...as is, of course the tradition) to check mismatches, clean them up and send emails, notifying the customers that the interval/booking has been cancelled.

Additional can of worms: What happens when the interval has been changed/deleted and customers no longer have booking permissions...should the dates be automatically extended?