The dismiss_sidebar_tutorial session action overrode the whole
URL, including replacing the base URL with just '/app'. Therefore
it would have failed when app.html's origin did not match the service
URL - as in the Chrome extension.

 * Pass the complete URL for the dismiss_sidebar_tutorial action

 * Remove several unused actions from session.js that have been
   replaced by server-rendered forms

 * Add test for dimiss_sidebar_tutorial action