Enable partial grading / save progress during grading

[IonMich]

Describe the bug
In assignments with a large number of questions, it is necessary to allow the user to partially grade submissions. This is also useful if a user wants to grade by question and not by submission. While an alert is already in place to warn the user before saving the partial progress, the actual "partial saving" functionality has not been implemented yet in JS and in the backend.

To Reproduce
Steps to reproduce the behavior:

1. Grade some (but not all) of the questions in a submission and try to navigate to previous/next submission.
2. In the alert that appears, choose save progress.
3. The alert disappears without saving the grade inputs and without navigating out of the page.

Additional Info
The fix could affect the calculation of grading_progress, average_score and which submissions are considered graded when attempting to upload to canvas. We probably need to treat graded as representing "fully-graded" submissions, and add functionality to identify the collection of partially graded submissions. Partially graded submissions should not affect the average score of the assignment, but it might make sense to affect the grading progress.

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