Change semantics of changing orders (#1260)

* Change semantics of changing orders

This basically does two things to the "Change products" view of orders and the
OrderChangeManager program API:

1) It decouples changing items or subevents from changing prices.
   OrderChangeManager.change_item() and .change_subevent() no longer
   touch the price of a position. Instead .change_price() needs to be
   called explicitly. However, a client-side JavaScript component now
   *proposes* a new price based on the changed item or subevent.

2) The user interface now exposes the possibility of doing multiple
   things at the same time, i.e. changing the item, subevent and price
   in the same operation. OrderChangeManager already allowed this
   before.

(1) is basically a consequence of (2), while (2) is a prerequesite for
e.g. the `seating` branch, where changing the subevent will always
require changing the seat.

* Add tests for price calculation API

src/pretix/api/serializers/order.py

@@ -14,8 +14,8 @@ from pretix.api.serializers.i18n import I18nAwareModelSerializer
 from pretix.base.channels import get_all_sales_channels
 from pretix.base.i18n import language
 from pretix.base.models import (
-    Checkin, Invoice, InvoiceAddress, InvoiceLine, Order, OrderPosition,
-    Question, QuestionAnswer,
+    Checkin, Invoice, InvoiceAddress, InvoiceLine, Item, ItemVariation, Order,
+    OrderPosition, Question, QuestionAnswer, SubEvent,
 )
 from pretix.base.models.orders import (
     CartPosition, OrderFee, OrderPayment, OrderRefund,
@@ -337,6 +337,23 @@ class OrderSerializer(I18nAwareModelSerializer):
         return instance
 
 
+class PriceCalcSerializer(serializers.Serializer):
+    item = serializers.PrimaryKeyRelatedField(queryset=Item.objects.none(), required=False, allow_null=True)
+    variation = serializers.PrimaryKeyRelatedField(queryset=ItemVariation.objects.none(), required=False, allow_null=True)
+    subevent = serializers.PrimaryKeyRelatedField(queryset=SubEvent.objects.none(), required=False, allow_null=True)
+    locale = serializers.CharField(allow_null=True, required=False)
+
+    def __init__(self, *args, **kwargs):
+        event = kwargs.pop('event')
+        super().__init__(*args, **kwargs)
+        self.fields['item'].queryset = event.items.all()
+        self.fields['variation'].queryset = ItemVariation.objects.filter(item__event=event)
+        if event.has_subevents:
+            self.fields['subevent'].queryset = event.subevents.all()
+        else:
+            del self.fields['subevent']
+
+
 class AnswerCreateSerializer(I18nAwareModelSerializer):
 
     class Meta: