Edx-proctoring

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* Fix a bug in exam attempt API where total time allowed for the exam would not include allowance time.
* Add `test_plan` document to describe key features and test cases
* Update `developing` document with the instructions for frontend-lib-special-exam local development setup

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* Created a POST api endpoint to add allowances for multiple students and multiple exams at the same time.

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* When an exam attempt is finished for the first time, mark all completable children in the exam as complete
in the Completion Service using the Instructor Service. If the Completion Service is not enabled, nothing
will happen.

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* Extend exam attempt API to return total time left in the attempt
and a link to the onboarding exam in case user tries to take proctored
exam when they haven't passed required onboarding exam.
Modify API to check if exam has passed due date.

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* If an attempt transitions from `ready_to_submit` back to `started`, the proctoring provider
backend function `start_exam_attempt` will not be called.

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* If the Django setting `PROCTORED_EXAM_VIEWABLE_PAST_DUE` is false, exam content will not be viewable past
an exam's due date, even if a learner has acknowledged their status.
* Extend exam attempt API to return exam type and to check if
user has satisfied prerequisites before taking proctored exam.
* Extend proctoring settings API to return additional data about proctoring
provider.
* Add API endpoint which provides exam review policy for specific exam.
Usage case is to provide required data for the learning app MFE.