An exam due date can be inferred from the end date of the course if the exam does not have a due date. In the legacy proctoring system (the edx-proctoring plugin), this inference is made in the proctoring code by calling the edx-when API. This is possible because edx-proctoring is a plugin that's installed into the edx-platform, into which edx-when is also installed.
In the new exams service, we do not want to call to the LMS to get due date information from edx-when. This poses a number of problems, not all of which are solved by this commit. This commit allows the exams service to infer a due date for an exam if that exam does not have a due date at the subsection level.
Note that this is a departure from edx-proctoring. This also opts out exams powered by the new edx-exams service from personalized learner schedules (PLS)/relative dates, because we no longer consider the pacing type of the course.
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