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/static/pretixcontrol/js/ui/main.js

@@ -573,6 +573,7 @@ var form_handlers = function (el) {
     el.find('[data-model-select2=generic]').each(function () {
         var $s = $(this);
         $s.select2({
+            closeOnSelect: !this.hasAttribute('multiple'),
             theme: "bootstrap",
             delay: 100,
             allowClear: !$s.prop("required"),