sort by activity

lms/templates/discussion/_sort.html

@@ -27,6 +27,8 @@
   Sort by:
   ${link_to_sort('date', 'date')}
   |
+  ${link_to_sort('activity', 'activity')}
+  |
   ${link_to_sort('votes', 'votes')}
   |
   ${link_to_sort('comments', 'comments')}