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/helpers/money.py

@@ -0,0 +1,34 @@
+from decimal import Decimal
+
+from django.conf import settings
+from django.core.validators import DecimalValidator
+from django.forms import NumberInput, TextInput
+from django.utils import formats
+
+
+class DecimalTextInput(TextInput):
+    def __init__(self, *args, **kwargs):
+        self.places = kwargs.pop('places', 2)
+        super().__init__(*args, **kwargs)
+
+    def format_value(self, value):
+        """
+        Return a value as it should appear when rendered in a template.
+        """
+        if value == '' or value is None:
+            return None
+        if isinstance(value, str):
+            return value
+        return formats.localize_input(value.quantize(Decimal('1') / 10 ** self.places))
+
+
+def change_decimal_field(field, currency):
+    places = settings.CURRENCY_PLACES.get(currency, 2)
+    field.decimal_places = places
+    if isinstance(field.widget, NumberInput):
+        field.widget.attrs['step'] = str(Decimal('1') / 10 ** places).lower()
+    elif isinstance(field.widget, TextInput):
+        field.widget = DecimalTextInput(places=places)
+    v = [v for v in field.validators if isinstance(v, DecimalValidator)]
+    if len(v) == 1:
+        v[0].decimal_places = places