Introduce a setting to show net prices (#415)

* Introduce a setting to show net prices in the frontend

* Show net prices in the backend as well

src/pretix/base/services/cart.py

@@ -9,6 +9,7 @@ from django.db.models import Q
 from django.utils.timezone import now
 from django.utils.translation import ugettext as _
 
+from pretix.base.decimal import round_decimal
 from pretix.base.i18n import LazyLocaleException
 from pretix.base.models import (
     CartPosition, Event, Item, ItemVariation, Voucher,
@@ -143,6 +144,8 @@ class CartManager:
                 custom_price = Decimal(custom_price.replace(",", "."))
             if custom_price > 100000000:
                 return error_messages['price_too_high']
+            if self.event.settings.display_net_prices:
+                custom_price = round_decimal(custom_price * (100 + item.tax_rate) / 100)
             price = max(custom_price, price)
 
         return price