n-tier questions in question set

It can be useful to have a question set allow n-tier questions.

A simple example is a question set that contains two multiple choice questions (2-tier question) with question 1 asking for a fact and question 2 asking for a justification why question 1 was correct. Passing the exercise could e.g. require to have all tiers answered correctly or to meet the passing requirements for each question.

It's easier to detect whether an answer was only correct by chance (for both, students and teachers). It's also easier to design more complex questions that prevent guessing the correct answer.

Some TODOs to achieve this in Question Set would be:

- Prevent students from skipping to next question before question was answered
- Prevent displaying the score for each question, but only use the summary
- Think about sensible scoring/passing mechanism and UX friendly implementation
- Override the "Show solution" and "Retry" button of questions (will require to do this for each single question otherwise)