DEV: Convert admin user fields to FormKit (#29070)

This change replaces the admin form for adding and editing custom user fields with a new FormKit implementation.

app/assets/javascripts/admin/addon/controllers/admin-user-fields.js

@@ -28,6 +28,7 @@ export default class AdminUserFieldsController extends Controller {
   createField() {
     const f = this.store.createRecord("user-field", {
       field_type: "text",
+      requirement: "optional",
       position: MAX_FIELDS,
     });
     this.model.pushObject(f);