🐛 [#5088] Ensure collapsed metadata fieldset is not hidden

When a lot of variables are selected, the metadata fieldset gets pushed down in the modal. If it is collapsed, this causes it to disappear and not be accessible at all. The CSS styling for this collapsed class contains an `overflow: hidden` property that causes this.

This change adds a custom scss class openforms-fieldset, which overrides it to `overflow: visible`

src/openforms/js/components/admin/form_design/registrations/json_dump/JSONDumpOptionsForm.js

@@ -65,6 +65,7 @@ const JSONDumpOptionsForm = ({name, label, schema, formData, onChange}) => {
               defaultMessage="Metadata variables"
             />
           }
+          extraClassName='openforms-fieldset'
           collapsible
           initialCollapsed
         >

src/openforms/scss/components/admin/_fieldset.scss

@@ -0,0 +1,9 @@
+/**
+ * The django admin `.collapsed` styling on fieldset elements sets `overflow: hidden;`.
+ * Seeing how we already hide the fieldset content in the javascript code,
+ * the `overflow: hidden;` only hides the fieldset toggle (which we want to keep visible)
+ */
+.openforms-fieldset.collapsed {
+  overflow: visible !important;
+}
+

src/openforms/scss/components/admin/_index.scss

@@ -19,6 +19,7 @@
 @import './column-field-value';
 @import './confirmation-email-template';
 @import './variablemapping';
+@import './fieldset';
 
 // Form design UI
 @import './select';