[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.

The issue does not name files or tests; start by locating the submission-handling entry point for tasks with multiple subproblems. Trace how completion of later questions is required, then verify that submitting the first subproblem no longer requires answers to subsequent questions.

Written by the indexing model from the issue text.