Rename JSX files with proper .jsx extension (#23733)

app/javascript/mastodon/features/ui/components/modal_loading.jsx

@@ -0,0 +1,20 @@
+import React from 'react';
+
+import LoadingIndicator from '../../../components/loading_indicator';
+
+// Keep the markup in sync with <BundleModalError />
+// (make sure they have the same dimensions)
+const ModalLoading = () => (
+  <div className='modal-root__modal error-modal'>
+    <div className='error-modal__body'>
+      <LoadingIndicator />
+    </div>
+    <div className='error-modal__footer'>
+      <div>
+        <button className='error-modal__nav onboarding-modal__skip' />
+      </div>
+    </div>
+  </div>
+);
+
+export default ModalLoading;