Add the ability to get to the next sequential question

What do you think about adding a link that you can click to get to the next sequential question? For instance, if you've just answered [the 305th question](https://cppquiz.org/quiz/question/305?result=US&answer=&did_answer=Answer), you can now click either on the "Next unanswered random question" button or the "Next sequential question" button (offhand names). The latter will lead you to [the 307th question](https://cppquiz.org/quiz/question/307), since there is no question numbered 306.

It might be useful in situations when you want to process all the questions one by one (e.g., you're cramming for a job interview or want to check existing question explanations before updating compiler explorer links :)). Currently, you can't just remember the last question you answered and continue where you left off the next day. And if you clear your local data on browser exit (nullify your session), it's hard to answer all the questions at all.

Start from the linked question URLs and the existing “Next unanswered random question” button to trace how question navigation currently works. Add a sequential-navigation option that moves from a question to the next existing question number, skipping gaps such as question 306, and verify that users can continue through questions in order.

Written by the indexing model from the issue text.