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/payment.py

@@ -72,7 +72,7 @@ from pretix.helpers.countries import CachedCountries
 from pretix.helpers.format import format_map
 from pretix.helpers.money import DecimalTextInput
 from pretix.multidomain.urlreverse import build_absolute_uri
-from pretix.presale.views import get_cart, get_cart_total
+from pretix.presale.views import get_cart
 from pretix.presale.views.cart import cart_session, get_or_create_cart_id
 
 logger = logging.getLogger(__name__)
@@ -1149,12 +1149,16 @@ class FreeOrderProvider(BasePaymentProvider):
         from .services.cart import get_fees
 
         cart = get_cart(request)
-        total = get_cart_total(request)
+
         try:
-            total += sum([f.value for f in get_fees(self.event, request, total, None, None, cart)])
+            fees = get_fees(event=request.event, request=request,
+                            invoice_address=None,
+                            payments=None, positions=cart)
         except TaxRule.SaleNotAllowed:
             # ignore for now, will fail on order creation
-            pass
+            fees = []
+        total = sum([c.price for c in cart]) + sum([f.value for f in fees])
+
         return total == 0
 
     def order_change_allowed(self, order: Order) -> bool: