Reference scriptaculous.js only from download page

scriptaculous.js is only actually used in the download page, so we only
reference it from there, instead of having it in the html_head include.

This saves us several HTTP requests per page (scriptaculous.js fetches
other js files by itself).
Add soft redirects from all download archive URLs

The previous website supported access to download archive pages both
with dot and dash-delimited version numbers: /download/archives/2.9.0/
and /download/archives/2-9-0/

GitHub pages now has the jekyll-redirect-from plugin, so we use it to
redirect from the dash-delimited URLs to the right page.