Implement grade weights + cumulative grade

[matt-lebl]

Frontend

A new weight field will be added to API responses alongside grades where appropriate. This will be a value between 0 and 1, representing the contribution of that item's grade to the total grade of the course. Using this value in combination with the grades of each item, calculate the total cumulative grade achieved to date. Where a grade item is listed but no grade is assigned, assume the grade is 0%.

Backend

Add weight field to each grade item, and return this weight field in API responses where the grade is returned.

[matt-lebl]

just added a new thing

{
    "comment": null,
    "itemName": "Quiz 2",
    "markDenominator": 5.0,
    "markNumerator": null,
    "percentage": null,
    "weight": 0.01
}

[stephenneale]

Added my implementation here, cumulative weights are calculated on loading the "Grades" page and displayed appropriately 😄