Scroll columns area to right when children property is changed (#4517)

The feature to pin column could hide the rightmost column, which is
specified with children property of ColumnsArea.

The user is likely to see the column when the property changed, so scroll
the area in such cases.

app/javascript/mastodon/components/column.js

@@ -1,7 +1,7 @@
 import React from 'react';
 import PropTypes from 'prop-types';
 import detectPassiveEvents from 'detect-passive-events';
-import scrollTop from '../scroll';
+import { scrollTop } from '../scroll';
 
 export default class Column extends React.PureComponent {