Improve page layout for printing and PDF export

[pya]

It is likely that some institutions want to print the Code of Conduct and/or the Sponsoring Terms & Conditions. Currently, the resulting PDF looks rather suboptimal. For example, there is no conference logo on the PDF. That means there is no indication what conference it actually is. Also the Twitter feed should not be on the page. The index page does not print at all in any sensible way. My CSS foo is not good enough to quickly fix this. I only know that we need to add some @media print but don't know what to do exactly. 😒