Resolve "Answer statistic/viewing answers"

Proposed changes

- Create view and logic for displaying answer-statistics:

• Learning status of the content-group in form of a progress bar
• Statistics for every content of a content group
  • Subject of the content
  • Total number of given answers
  • Number of abstentions
  • Percentage of correct answers given
• Progress bar and list are colored according to their status

Closes #177 (closed)