ZIP Imports: Updated import form to show loading indicator

And disable button after submit.
Added here because the import could take some time, so it's best to show
an indicator to the user to show that something is happening, and help
prevent duplicate submission or re-submit attempts.

resources/js/components/index.js

@@ -30,6 +30,7 @@ export {HeaderMobileToggle} from './header-mobile-toggle';
 export {ImageManager} from './image-manager';
 export {ImagePicker} from './image-picker';
 export {ListSortControl} from './list-sort-control';
+export {LoadingButton} from './loading-button';
 export {MarkdownEditor} from './markdown-editor';
 export {NewUserPassword} from './new-user-password';
 export {Notification} from './notification';