Fix infinite scrolling in Pages

fixes RCX-2192
flag=top_navigation_placement

Test Plan:
- Have FF on
- Have 60+ Pages in a course
- Load course > go to Pages
- Scroll down
* It keeps loading pages as you scrolling

- Have FF off
- Infinite scroll still works

Change-Id: I072815a43e73527a14303fb3712d3fb8b47d6830
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/355352
Reviewed-by: Eric Saupe <eric.saupe@instructure.com>
QA-Review: Eric Saupe <eric.saupe@instructure.com>
Product-Review: Jacob DeWar <jacob.dewar@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

ui/shared/pagination/backbone/views/PaginatedCollectionView.jsx

@@ -57,7 +57,7 @@ PaginatedCollectionView.prototype.defaults = {
   buffer: 500,
   // Container with observed scroll position, can be a jQuery element, raw
   // dom node, or selector
-  scrollContainer: window,
+  scrollContainer: document.getElementById('drawer-layout-content') || window,
 }
 
 // Adds a loading indicator element