Tournament leaderboards should report attainable coverage

I.e. the coverage column should be sum(coverage) / sum(attainable_coverage) over all questions, including the ones you didn't forecast (where your coverage is 0).

Where attainable_coverage = (effective_close_time - open_time) / (scheduled_closed_time - open_time).

The "My score" section also needs some extra info to explain things:

• the "Coverage" column becomes "Coverage (max)", where each row shows your coverage, and the maximum attainable coverage in parantheses: "58% (84%)".
• the totals at the bottom have both:
  • "total coverage" = avg(coverage)
  • "total attainable coverage" = avg(attainable_coverage)
  • "effective coverage" = sum(coverage) / sum(attainable_coverage), i.e. should be the same as the value in the leaderboard.