Close #24. Add Attendance Leaderboard

Completely untested but I think this will work (might even be able to refactor that 2nd line more)
```
attendances_for_event_type = object.attendances.select do |attendance|
  a.present && Event.find(a.event_id).event_type_id == type.id
end
attendances_for_event_type.count
```