reorganize PageEditor layout

src/client/js/components/PageEditor.jsx

@@ -307,7 +307,7 @@ class PageEditor extends React.Component {
             onSave={this.onSaveWithShortcut}
           />
         </div>
-        <div className="d-none d-xl-block page-editor-preview-container flex-grow-1 flex-basis-0 mw-0">
+        <div className="d-none d-lg-block page-editor-preview-container flex-grow-1 flex-basis-0 mw-0">
           <Preview
             markdown={this.state.markdown}
             // eslint-disable-next-line no-return-assign