fix: Autoscroll moves element to center of page

This PR fixes the anchor tag's position on the page when autoscrolling
is used. Previously, the scroll would move the element to the center of
the page. Now the scroll moves the element to the top of the page. The
only case where the element will not be at the top of the page is when
the element is too close to the bottom of the page and there is not
enough page remaining to force the element to the top.

src/courseware/course/sequence/Unit.jsx

@@ -139,7 +139,7 @@ function Unit({
       } else if (event.data.offset) {
         // We listen for this message from LMS to know when the page needs to
         // be scrolled to another location on the page.
-        window.scrollTo(0, event.data.offset);
+        window.scrollTo(0, event.data.offset + document.getElementById('unit-iframe').offsetTop);
       }
     }
     // If we currently have an event listener, remove it.