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

app/javascript/mastodon/components/column_back_button_slim.jsx

@@ -0,0 +1,19 @@
+import React from 'react';
+import { FormattedMessage } from 'react-intl';
+import ColumnBackButton from './column_back_button';
+import Icon from 'mastodon/components/icon';
+
+export default class ColumnBackButtonSlim extends ColumnBackButton {
+
+  render () {
+    return (
+      <div className='column-back-button--slim'>
+        <div role='button' tabIndex='0' onClick={this.handleClick} className='column-back-button column-back-button--slim-button'>
+          <Icon id='chevron-left' className='column-back-button__icon' fixedWidth />
+          <FormattedMessage id='column_back_button.label' defaultMessage='Back' />
+        </div>
+      </div>
+    );
+  }
+
+}