Add prioritization marker to hilight important papers

[willscott]

trafficstars

There's currently no indication of the most important papers. anonbib has categories, and it might be worth having some way to highlight papers that are particularly important in mixnet thought

[david415]

probably all the papers written by George Danezis fall into the highlight category. and a few written by Paul Syverson... like "Sleeping dogs lie on a bed of onions but wake when mixed" or "Why I'm not an entropist".