make top sort by last_activity_at for now

lms/templates/discussion/_sort.html

@@ -25,7 +25,7 @@
 
 <div class="discussion-sort discussion-local">
   <span class="discussion-label">Sort by:</span>
-  ${link_to_sort('top', 'top')}
+  ${link_to_sort('activity', 'top')}
   
   ${link_to_sort('date', 'date')}