Fixed flow errors and small refactor

frontend/components/DocumentPreview/DocumentPreview.js

@@ -8,7 +8,7 @@ import PublishingInfo from 'components/PublishingInfo';
 
 type Props = {
   document: Document,
-  highlight?: string,
+  highlight?: ?string,
   innerRef?: Function,
 };
 

frontend/components/Layout/components/Title/Title.js

@@ -15,7 +15,7 @@ class Title extends React.Component {
   render() {
     let title;
     if (this.props.truncate) {
-      title = _.truncate(this.props.content, this.props.truncate);
+      title = _.truncate(this.props.content, { length: this.props.truncate });
     } else {
       title = this.props.content;
     }