Prevent double-booking when using "force allow"

The "Force Allow" override is a sharp tool! Previously, "force allow" would allow anyone to schedule in the range, even if someone booked a time using one of your scheduling links. This means it was possible for the same slot to get booked multiple times 👎

We've fixed that behavior, so now "force allow" will no longer apply to events that were created in SavvyCal.