(Bug 4721) Whitelist Goodreads updates widget

The Goodreads updates widget uses an iframe, which we're now whitelisting.

The Goodreads "my books" widget uses flash; this will work without further
effort on our part.

All other widgets we're aware of use JS, or are a mix of dynamic and 
non-dynamic content. The non-dynamic content will work fine, without any 
further action on our part.