Improve focus management of forums nav sidebar

Now, when the list is reset (e.g. by sorting), the first thread is
focused. When the "Load more" link is clicked, upon loading more
threads, focus moves to the thread immediately following the thread
that was previously last in the list.

common/static/coffee/src/discussion/views/discussion_thread_list_view.coffee

@@ -144,6 +144,18 @@ if Backbone?
             options.group_id = @group_id
         
     
+      lastThread = @collection.last()?.get('id')
+      if lastThread
+        # Pagination; focus the first thread after what was previously the last thread
+        @once("threads:rendered", ->
+          $(".post-list li:has(a[data-id='#{lastThread}']) + li a").focus()
+        )
+      else
+        # Totally refreshing the list (e.g. from clicking a sort button); focus the first thread
+        @once("threads:rendered", ->
+          $(".post-list a").first()?.focus()
+        )
+
       @collection.retrieveAnotherPage(@mode, options, {sort_key: @sortBy})
 
     renderThread: (thread) =>