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.
HTML bug fix on the blob page

That blob page sent *two* body tags. Now that we have bottom scripts,
we can fix that easily: don't try to set body.onload, but run the
prettyprinting through a bottom script on jQuery's document.ready.
Opacity adjustments for image diffs

* ImageDiffHandler adds the slider; styled in gitblit.css
* imgdiff.js is a little bottom-loaded Javascript that adjusts the
  opacity on sliders' scroll events.
* The three diff pages add this bottom script to the page if needed
* GitBlitDiffFormatter: center image diffs.