[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.

该 issue 没有指定文件或测试；首先定位处理包含多个子问题的任务提交的入口点。跟踪后续问题的完成是如何被要求的，然后验证提交第一个子问题时不再需要回答后续问题。

由索引模型根据 Issue 内容生成。