Leaderboard element; styled.

What started out as simple CSS ended up becoming ...more:

* Supporting multiple "focus row" moves at a time was really distracting/confusing,
  so this clamps to one at a time (highest rank first).

* Switching over to web animations to make use of all the timing & events.

* Sticking with a single set of rows, rather than the duplicates so that the fade
  out step of the animation sequence can be more fluid.
Merge pull request #28 from nevir/leaderboard-elem

Leaderboard element; styled.