Fix relation between MultipleChoiceOption and FeedbackOption

When a MultipleChoiceOption is created via the API call PUT /mult-choice, a new FeedbackOption is now automatically inserted into the database that links to the same problem as the MultipleChoiceOption.

The response of PUT /mult-choice will now contain an mc_id that shows the ID of the new MultipleChoiceQuestion and a feedback_id with the ID of the new FeedbackOption.

The relation between MultipleChoiceOption and Problem is now removed. A new relation from FeedbackOption to MultipleChoiceOption is created. However, it is still possible to use problem.mc_options with a hybrid attribute that essentially runs a query in the background.

TODO:

• Ensure that if a FeedbackOption is deleted, the related MultipleChoiceOption is also deleted.

Closes #35 (closed) and #36 (closed).