Quota form: Change item selection field per context (#3839)

* Change item checkbox select to multiselect widget

* Make item selection widget dependent on count

* Make item selection widget dependent on variable

* Adjust widget choices

* Fix widget choices

* Fix item variation key errors

* Simplify code

* Fix classname

* Improve argument name

* Fix widget name

src/pretix/control/forms/widgets.py

@@ -77,3 +77,19 @@ class Select2ItemVarQuotaMixin(Select2Mixin):
 
 class Select2ItemVarQuota(Select2ItemVarQuotaMixin, forms.Select):
     pass
+
+
+class Select2ItemVarMulti(Select2Mixin, forms.SelectMultiple):
+    def options(self, name, value, attrs=None):
+        # we need this for multi-selection without a queryset for the selection of items and variations
+        for i, v in enumerate(value):
+            yield self.create_option(
+                None,
+                v,
+                dict(self.choices)[v],
+                True,
+                i,
+                subindex=None,
+                attrs=attrs
+            )
+        return