Refs #99 -- Improve support for currencies with less than 2 decimal places (#783)

* Refs #99 -- Fix stripe support for zero-decimal currencies

* Add new money formatting method

* Force decimal places in many places

* Locale-aware currency rendering

* Fix currencies in more places

* More currency fixes

src/pretix/base/payment.py

@@ -1,10 +1,11 @@
 import logging
 from collections import OrderedDict
-from decimal import Decimal
+from decimal import ROUND_HALF_UP, Decimal
 from typing import Any, Dict, Union
 
 import pytz
 from django import forms
+from django.conf import settings
 from django.contrib import messages
 from django.dispatch import receiver
 from django.forms import Form
@@ -15,11 +16,11 @@ from django.utils.translation import pgettext_lazy, ugettext_lazy as _
 from i18nfield.forms import I18nFormField, I18nTextarea
 from i18nfield.strings import LazyI18nString
 
-from pretix.base.decimal import round_decimal
 from pretix.base.models import CartPosition, Event, Order, Quota
 from pretix.base.reldate import RelativeDateField, RelativeDateWrapper
 from pretix.base.settings import SettingsSandbox
 from pretix.base.signals import register_payment_providers
+from pretix.helpers.money import DecimalTextInput
 from pretix.presale.views import get_cart_total
 from pretix.presale.views.cart import get_or_create_cart_id
 
@@ -91,10 +92,15 @@ class BasePaymentProvider:
         fee_abs = self.settings.get('_fee_abs', as_type=Decimal, default=0)
         fee_percent = self.settings.get('_fee_percent', as_type=Decimal, default=0)
         fee_reverse_calc = self.settings.get('_fee_reverse_calc', as_type=bool, default=True)
+        places = settings.CURRENCY_PLACES.get(self.event.currency, 2)
         if fee_reverse_calc:
-            return round_decimal((price + fee_abs) * (1 / (1 - fee_percent / 100)) - price)
+            return ((price + fee_abs) * (1 / (1 - fee_percent / 100)) - price).quantize(
+                Decimal('1') / 10 ** places, ROUND_HALF_UP
+            )
         else:
-            return round_decimal(price * fee_percent / 100) + fee_abs
+            return (price * fee_percent / 100 + fee_abs).quantize(
+                Decimal('1') / 10 ** places, ROUND_HALF_UP
+            )
 
     @property
     def verbose_name(self) -> str:
@@ -156,6 +162,7 @@ class BasePaymentProvider:
         .. WARNING:: It is highly discouraged to alter the ``_enabled`` field of the default
                      implementation.
         """
+        places = settings.CURRENCY_PLACES.get(self.event.currency, 2)
         return OrderedDict([
             ('_enabled',
              forms.BooleanField(
@@ -166,7 +173,10 @@ class BasePaymentProvider:
              forms.DecimalField(
                  label=_('Additional fee'),
                  help_text=_('Absolute value'),
-                 required=False
+                 localize=True,
+                 required=False,
+                 decimal_places=places,
+                 widget=DecimalTextInput(places=places)
              )),
             ('_fee_percent',
              forms.DecimalField(
@@ -174,7 +184,8 @@ class BasePaymentProvider:
                  help_text=_('Percentage of the order total. Note that this percentage will currently only '
                              'be calculated on the summed price of sold tickets, not on other fees like e.g. shipping '
                              'fees, if there are any.'),
-                 required=False
+                 localize=True,
+                 required=False,
              )),
             ('_availability_date',
              RelativeDateField(