Event cancellation: Add safety and security checks (#5565)

* Event cancellation: Add safety and security checks

When cancelling an event, a large sum of money might be refunded
instantly. This PR adds safety features around this by

- doing a dry-run first that shows a preview of the expected refund sum

- sending a confirmation mode via email for any automatic refunds of more than 100 currency units

- keeping a more detailed log of the settings this was executed with

* Update src/pretix/control/views/orders.py

Co-authored-by: luelista <weller@rami.io>

---------

Co-authored-by: luelista <weller@rami.io>

src/pretix/control/urls.py

@@ -460,6 +460,7 @@ urlpatterns = [
         re_path(r'^orders/search$', orders.OrderSearch.as_view(), name='event.orders.search'),
         re_path(r'^dangerzone/$', event.DangerZone.as_view(), name='event.dangerzone'),
         re_path(r'^cancel/$', orders.EventCancel.as_view(), name='event.cancel'),
+        re_path(r'^cancel/(?P<task>[^/]+)/$', orders.EventCancelConfirm.as_view(), name='event.cancel.confirm'),
         re_path(r'^shredder/$', shredder.StartShredView.as_view(), name='event.shredder.start'),
         re_path(r'^shredder/export$', shredder.ShredExportView.as_view(), name='event.shredder.export'),
         re_path(r'^shredder/download/(?P<file>[^/]+)/$', shredder.ShredDownloadView.as_view(), name='event.shredder.download'),