Rework Dingus to use Front Controller

* Keep logic out of the JSP templates, as far as possible.
* Fix drop down "Results" menu to be "sticky".
* Based on Heroku's "Containerless web app" application template.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Tell App Engine to allow Concurrent Requests

Signed-off-by: Alex Coles <alex@alexbcoles.com>