Small refactor to be more React-y

The selected member of a collection is part of the state of that collection. The collection not the unit should then be responsible for selecting. Benefits, Meteor won't need to auto call `setState` on each `Player` component now. Instead the leaderboard collection component will try to rerender each `Player` and React can prune away anything that hasn't changed.

Also changed `<Player id=` to `<Player key=` as mentioned in the React docs.