Moved reference to throbber.gif to CSS file.

In a more complex set-up, a `swagger-ui` page may be served deeper into the site wherein `img.response_throbber` cannot locate `../images/throbber.gif`, because it simply isn't there.

This patch moves the reference to the gif from the `img` tag and into the CSS file. Therefore the image only needs to be located relative to the CSS file, not on the page itself.

(This is somewhat related to https://github.com/wordnik/swagger-ui/issues/470)