fix: xblock error mfe unit preview (#1508)

* feat: add functionality to see unit draft preview

* fix: course redirect unit to sequnce unit redirect

* fix: not showing preview when masquerading

* feat: in preview fetch draft branch of sequence metadata

src/courseware/CoursewareContainer.jsx

@@ -168,7 +168,7 @@ class CoursewareContainer extends Component {
 
   checkFetchSequence = memoize((sequenceId) => {
     if (sequenceId) {
-      this.props.fetchSequence(sequenceId);
+      this.props.fetchSequence(sequenceId, this.props.isPreview);
     }
   });