[New feature] Single subproblem submission

Some tasks may include multiple subproblems such as, for example, a mathematical problem followed by a MCQ talking about the math problem answer. If the student wants to submit only the first subproblem, he needs to answer to the following MCQ or it will not be submitted. It would be better to allow only first subproblem submission.
Current fix: The student answers randomly to the following questions just to submit the first subproblem.