Redesign: Improve Grading Pipeline

[geshuming]

Just to log the issue down first.

The current pipeline:

• Submission does not exist
• Student submits at least 1 answer, submission is created, marked as attempting
• Student answers all the questions, submission marked as attempted
• Student betcha's, submission marked as submitted, autograder invoked.
• At 3am, regardless of submission status, a submission is created where there is none, and all submissions marked as submitted, autograder invoked regardless of status.
• Manual grading should be done after this stage, or the grade adjustment is reset.

Proposed pipeline: Use Not Attempted, Attempted, Submitted for submission status, as discussed in https://github.com/source-academy/cadet-frontend/issues/738

• Submission does not exist.
• Student submits at least 1 answer, submission is created, marked as attempted
• Student answers all the questions
• Student betcha's, submission marked as submitted, autograder invoked.
• At 3am, regardless of submission status, a submission is created where there is none and marked as Not Attempted. autograder invoked for non-Submitted submissions and the corresponding graded submissions marked as Submitted
• Manual grading can be done at any stage after the submission is marked as Submitted

[martin-henz]

As discussed with Shuming, let's postpone this one until December 2019.