Fix formset widths (#5530)

* Remove explicitly specified width for formset-forms
With that style, all formset rows were a fix pixels less wide than surrounding content

* Set select2 width to 100% so they adapt when browser window is resized

src/pretix/static/pretixcontrol/js/ui/main.js

@@ -485,6 +485,7 @@ var form_handlers = function (el) {
             theme: "bootstrap",
             language: $("body").attr("data-select2-locale"),
             data: JSON.parse($(this.getAttribute('data-select2-src')).text()),
+            width: '100%',
         }).val(selectedValue).trigger('change');
     });