enable user to dismiss multiple tasks at a time

The next step is a "select all" button, followed by dismissable
alert boxes, but one thing at a time.
remove datetime objects from Celery tasks when dismissing

bc datetime objects are not serializable into JSON.
select-all button for task-dismiss list
refresh tasks-list page once per minute
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
Merge branch '172-task-dismiss-ui'