get more events on page down or up when needed

when we page up or down, check if we've moved into the upper or lower
threshold of the scrollbar and request more events to show if we have.

awx/ui/client/features/output/index.controller.js

@@ -422,10 +422,18 @@ function menuLast () {
 
 function down () {
     scroll.moveDown();
+
+    if (scroll.isBeyondLowerThreshold()) {
+        next();
+    }
 }
 
 function up () {
     scroll.moveUp();
+
+    if (scroll.isBeyondUpperThreshold()) {
+        previous();
+    }
 }
 
 function togglePanelExpand () {