👽 [#4969] Got bootstrap'ed again

Bootstrap CSS hides the django collapsed fieldsets because no .show
class is present. The easiest workaround is to add this class name to
ignore bootstrap's behaviour.

src/openforms/js/components/admin/forms/Fieldset.js

@@ -63,7 +63,7 @@ const Fieldset = ({
     </h2>
   ) : null;
   const className = classNames('module', 'aligned', extraClassName, {
-    'collapse in': collapsible,
+    'collapse in show': collapsible,
     collapsed: collapsed,
   });