fixes #2386 update filters in users activity logs (#2399)

* Add an action filter and a date filter. 

* Additional filters on a specific object (photo/album/group) are available from their dedicated administration page.

* Performances of the page was improved : instead of loading 100k lines in activity table, we loop on 500 activity lines until 100 aggregated lines are found for the current page.

admin/group_list.php

@@ -102,6 +102,8 @@ SELECT u.'. $conf['user_fields']['username'].' AS username
       )
     );
 
+  $template->assign('U_ACTIVITY', get_root_url().'admin.php?page=user_activity&group='.$row['id']);
+
   $group_counter++;
 }