One rating per problem and new Arena 90 art

What changed

• Each problem now shows one rating everywhere on the site, and its easy, medium, or hard label comes from that number. Problems that used to show medium by default now get a real difficulty.
• The problem library's Companies column shows up to three company logos per problem in place of two cut-off names.
• The Arena 90 campaign map was redrawn, with new art for the overview and each of its nine regions.