Make 'date' the first sort key since it's the new default (students seem to…

Make 'date' the first sort key since it's the new default (students seem to prefer it, especially since top is kind of unclear).

lms/templates/discussion/_sort.html

@@ -31,9 +31,9 @@
 
 <div class="discussion-sort local">
   <span class="discussion-label">Sort by:</span>
-  ${link_to_sort('activity', 'top')}
-  
   ${link_to_sort('date', 'date')}
+
+  ${link_to_sort('activity', 'top')}
   
   ${link_to_sort('votes', 'votes')}