Expand on the maintenance code to make it more robust and give the

admin more control.  You can now track running tasks, resume stalled
tasks, cancel running tasks, and remove finished tasks.

Added graphics::compose() as a placeholder for future watermark
operations.

Added CSRF protection to maintenance urls.

core/views/admin_maintenance_task.html.php

@@ -3,7 +3,7 @@
 <script type="text/javascript">
   update = function() {
     $.ajax({
-      url: "<?= url::site("admin/maintenance/run/$task->id") ?>",
+      url: "<?= url::site("admin/maintenance/run/$task->id?csrf=$csrf") ?>",
       dataType: "json",
       success: function(data) {
         $("#gStatus").html("" + data.task.status);