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/base/templates/pretixbase/email/cancel_confirm.txt

@@ -0,0 +1,10 @@
+{% load i18n %}
+{% trans "You have requested us to cancel an event which includes a larger bulk-refund:" %}
+
+{% trans "Event" %}: {{ event }}
+
+{% trans "Estimated refund amount" %}: **{{ amount }}**
+
+{% trans "Please confirm that you want to proceed by coping the following confirmation code into the cancellation form:" %}
+
+**{{ confirmation_code }}**