Refine answer statistics

There are some new features and changes for the list of answers and the detailed view of the answer statistic:

Answer-list

• there should be a legend in which the colors are explained
• the scale should be absolute instead of percentage

Detailed view

• abstentions should be displayed as bar in chart
• you should be able to mark the correct answer
• answers which not has been answered until now should be not clickable
• progress bar with current collection status should be only displayed when already answered