Implemented basic quiz viewing and question submission

- Users can now take quizzes (in addition to editing them).
- Users can submit questions and get basic feedback ("Correct" or "Incorrect", along with the correct answer).
- Implemented a basic sidebar for navigating questions within a quiz.

What's still left:
- At this point, no score is kept, and there is no feedback about progress through the quiz.
- Nothing happens yet when you complete the quiz.
- Questions may be submitted multiple times - there is no cheat prevention (we're not keeping track of quiz state on the server yet).