Allow to round taxes on order-level (#5019)

* Allow to round taxes on order-level

* Rename get_cart_total

* Persist rounding mode with order

* Add general docs

* Order creation API

* Update fee algorithm

* Rounding on payment method change

* Round when splitting order

* Fix failing tests

* Add settings page

* Add tests

* Replace algorithm

* Add test case for currency rounding

* Improve order change

* Update flowchart

* Update discount logic (more hypothetical, we don't store rounding on cart positions atm)

* Rename internal method

* Fix typo

* Update help text

* Apply suggestions from code review

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

* Order rounding refactor (#5571)

* Add RoundingCorrectionMixin providing before-rounding-values as properties

* Use gross_price_before_rounding in more places

* Update doc/development/algorithms/pricing.rst

Co-authored-by: Martin Gross <gross@rami.io>

* Allow to override on perform_order

* Rebase migration

* Fix event cancellation

---------

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

src/pretix/base/services/cancelevent.py

@@ -350,7 +350,7 @@ def cancel_event(self, event: Event, subevent: int, auto_refund: bool,
                     ocm.add_fee(f)
 
             if dry_run:
-                refund_total += max(payment_refund_sum - (o.total + ocm._totaldiff), Decimal("0.00"))
+                refund_total += max(payment_refund_sum - (o.total + ocm._totaldiff_guesstimate), Decimal("0.00"))
             else:
                 ocm.commit()
                 refund_amount = payment_refund_sum - o.total