Fix issue where arrowing-down near end of page would skip a message.

We had this fascinating behavior where pressing a down arrow near the
end of the page would advance the pointer, call recenter_view, which
would trigger a scroll event, which would call keep_pointer_in_view,
which would notice that we were at the end of the page and advance the
pointer again!

I split out that last part into its own function which is only called
on mousewheel events.

(imported from commit bc85443e762356e3055f8f88585940a1f11f9124)

tools/jslint/check-all.js

@@ -39,7 +39,8 @@ var globals =
     // zephyr.js
     + ' message_array message_dict'
     + ' status_classes clear_table add_to_table instance_list'
-    + ' keep_pointer_in_view respond_to_message'
+    + ' keep_pointer_in_view move_pointer_at_page_top_and_bottom'
+    + ' respond_to_message'
     + ' select_message select_message_by_id'
     + ' scroll_to_selected select_and_show_by_id'
     + ' selected_message selected_message_id'