Integrate leaderboard for assignment

• floating window to show people with the fewest hours
• add a leaderboard button to each session, when clicked: filter floating leaderboard with cadidates for this session
• sort candidates by reverse position on the leaderboard
• sum of hours could be one thing; OTOH, the number of assignments (show count for the current day, also show count for the whole event) could also be interesting values to display