Add prioritization marker to hilight important papers

[willscott]

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".