Support for adding bottom scripts

Needed if we want to have opacity changes in image diffs because
jQuery is bottom-loaded, so we must ensure that any scripts using
jQuery are run later.

I'm not a Wicket expert; maybe there's a cleverer or cleaner way to do
this. There is a JavascriptUtils class in Wicket, but that writes to
the response -- I don't quite see how that would give me control over
the precise placement of the scripts to ensure they come after that
bottom-loaded jQuery.