update scoreboard for better ux

[cynthiarich]

Yay! The scoreboard now behaves as follows:

• all users show up on the scoreboard all the time
• when a question is first displayed, the user opacity is set to 50%
• when a user has responded, the user opacity changes to 100%
• when we score the questions, the user moves to their new position on the board

The only remaining gotcha is the first question - which we can only solve by adding null as a value on the quizScores table;