move ChoiceAnswer model to folder

[lebalz]

Plan

• ChoiceAnswers always have either an explicite id (uuid) when standalone or a qid when inside q Quiz
• Create documents for each Quiz-Item:
  • having a prop qid when inside a Quiz (to ensure ordering)
  • attach to the Quiz' DocumentRoot or to it's own DocumentRoot when standalone
• create an interface iQuizzable providing all needed methods to assess quiz