Move the rendering of featured images to jQuery using `data-` attributes

This simplifies both the PHP *and* the JavaScript, somewhat:

* In PHP, we don't have to iterate through the list twice, and
* In JavaScript, we don't have to do as much DOM lookup.

Unfortunately the `render()` method puts HTML into our JS, where it is less than welcome!
Merge pull request #1 from kadamwhite/jquery-with-data-attrs

Move the rendering of featured images to jQuery using `data-` attributes