POSTs the page URL to startupdigest's url.
Uses iframe src attribute instead of an ajax POST.

Using a POST had two drawbacks:

1) The original request could not be stored in the session, so that the
   user was redirected on success.
2) Without src set on the iframe, the frame had no idea where to go for
   assets located by relative path.

Despite using a GET request to create a resource being non-REST-ful,
it's a much simpler, cleaner solution.