Long overdue updates

Most of it is invisible as the internals and the content generation scripts have been upgraded and cleaned up a bit, but it now lets me modify the site more reliably.

CDF documents are now links to download, since they don’t run on the browser anymore. Embedded YouTube videos should appear on some of the pages as I create them. Time has become vague on the site, new content is highlighted, old content is just marked as some time ago, since most of the content is lecture material this seems to fit better. Also the site is now available with https.