Add sub-events and relative date settings (#503)

* Data model

* little crud

* SubEventItemForm etc

* Drop SubEventItem.active, quota editor

* Fix failing tests

* First frontend stuff

* Addons form stuff

* Quota calculation

* net price display on EventIndex

* Add tests, solve some bugs

* Correct quota selection in more places, consolidate pricing logic

* Fix failing quota tests

* Fix TypeError

* Add tests for checkout

* Fixed a bug in QuotaForm

* Prevent immutable cart if a quota was removed from an item

* Add tests for pricing

* Handle waiting list

* Filter in check-in list

* Fixed import lost in rebase

* Fix waiting list widget

* Voucher management

* Voucher redemption

* Fix broken tests

* Add subevents to OrderChangeManager

* Create a subevent during event creation

* Fix bulk voucher creation

* Introduce subevent.active

* Copy from for subevents

* Show active in list

* ICal download for subevents

* Check start and end of presale

* Failing tests / show cart logic

* Test

* Rebase migrations

* REST API integration of sub-events

* Integrate quota calculation into the traditional quota form

* Make subevent argument to add_position optional

* Log-display foo

* pretixdroid and subevents

* Filter by subevent

* Add more tests

* Some mor tests

* Rebase fixes

* More tests

* Relative dates

* Restrict selection in relative datetime widgets

* Filter subevent list

* Re-label has_subevents

* Rebase fixes, subevents in calendar view

* Performance and caching issues

* Refactor calendar templates

* Permission tests

* Calendar fixes and month selection

* subevent selection

* Rename subevents to dates

* Add tests for calendar views

src/pretix/api/views/item.py

@@ -58,10 +58,17 @@ class QuestionViewSet(viewsets.ReadOnlyModelViewSet):
         return self.request.event.questions.prefetch_related('options').all()
 
 
+class QuotaFilter(FilterSet):
+    class Meta:
+        model = Quota
+        fields = ['subevent']
+
+
 class QuotaViewSet(viewsets.ReadOnlyModelViewSet):
     serializer_class = QuotaSerializer
     queryset = Quota.objects.none()
-    filter_backends = (OrderingFilter,)
+    filter_backends = (DjangoFilterBackend, OrderingFilter,)
+    filter_class = QuotaFilter
     ordering_fields = ('id', 'size')
     ordering = ('id',)