refresh tasks-list more often; better messaging

- Refreshes every 30sec instead of every minute.
- Slight wording changes.
- Explanation if no tasks plus a "Back" button.
task_dismiss view for use in AJAXy task dismissal
TaskDismissForm now correctly handles empty tasks list

Formerly this would error out once there were no more tasks.
enable AJAX-y dismissal of celery tasks from inside alert box