Add Subevent-Filter for Voucher-Tags (#1407)

* Add Subevent-Filter for Voucher-Tags

* Filter Subevent Voucher-Tags with proper Filter

* Apply filter before annotating totals and usage

src/pretix/control/forms/filter.py

@@ -902,6 +902,43 @@ class VoucherFilterForm(FilterForm):
         return qs
 
 
+class VoucherTagFilterForm(FilterForm):
+    subevent = forms.ModelChoiceField(
+        label=pgettext_lazy('subevent', 'Date'),
+        queryset=SubEvent.objects.none(),
+        required=False,
+        empty_label=pgettext_lazy('subevent', 'All dates')
+    )
+
+    def __init__(self, *args, **kwargs):
+        self.event = kwargs.pop('event')
+        super().__init__(*args, **kwargs)
+
+        if self.event.has_subevents:
+            self.fields['subevent'].queryset = self.event.subevents.all()
+            self.fields['subevent'].widget = Select2(
+                attrs={
+                    'data-model-select2': 'event',
+                    'data-select2-url': reverse('control:event.subevents.select2', kwargs={
+                        'event': self.event.slug,
+                        'organizer': self.event.organizer.slug,
+                    }),
+                    'data-placeholder': pgettext_lazy('subevent', 'All dates')
+                }
+            )
+            self.fields['subevent'].widget.choices = self.fields['subevent'].choices
+        elif 'subevent':
+            del self.fields['subevent']
+
+    def filter_qs(self, qs):
+        fdata = self.cleaned_data
+
+        if fdata.get('subevent'):
+            qs = qs.filter(subevent_id=fdata.get('subevent').pk)
+
+        return qs
+
+
 class RefundFilterForm(FilterForm):
     provider = forms.ChoiceField(
         label=_('Payment provider'),