Dates are vanished and re created during import.
Slugs for dates were generate after whole import was finished.
This is now changed to generate slugs for dates after saving each single
event.
A new TypoScript option upcoming is added.
The option can be set to 0 (default) or 1.
0 behaves the same way as in the past.
1 turns off the option useMidnight, start and end.
Only dates with a start date in the future will be shown.
Relates: #10507
