Google Books Preview rewrite
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The Google Books Preview functionality in record detail pages has been
rewritten to modernize its style and optimize its performance:

* The Dojo JavaScript framework is no longer used, saving approximately
  150K of JavaScript and CSS and four network requests per page load.
* The Embedded Viewer is not loaded unless a possible preview is found,
  saving more network and memory overhead.
* The Google Books Loader is used to load the Embedded Viewer instead of the
  https://productforums.google.com/forum/#!topic/books-api/lZrq5cWKrTo;context-place=forum/books-api[deprecated
  Google Loader].
* All variables are self-contained and do not pollute the global namespace.
* Event listeners are registered to handle clicks, rather than attaching
  `href="javascript:function()"` to <a> elements.
* Book previews are displayed in a panel sized according to the viewport
  of the browser, improving its appearance on both mobile and desktop
  browsers.
* The rewritten code is now served up directly from
  `/js/ui/default/opac/ac_google_books.js` rather than as a TT2 template.