append javascript after CSS
Javascript resources should be presented to browsers after CSS, and
"after the fold" (ATF) according to the best practices:
https://developers.google.com/speed/docs/insights/mobile#PutStylesBeforeScripts
This change allows the browser to download Javascript files in
parallel, by including Javascript on the *closing* </body> tag instead
of the opening tag.
We also improve the regex to tolerate spaces before the body tag, as
some templates have (proper) indentation for the tag.