Overhaul grading states design

[chownces]

The current grading states are insufficient to represent all possible grading states. For example,

Student submits assessment --> avenger grades all questions --> avenger unsubmits assessment (for student to make changes) --> student resubmits

Issues:

1. After the avenger first grades all questions, the results are immediately shown to the student
2. When the student resubmits, the assessment is immediately 'Graded' due to point 1 above. This is a source of confusion for both the student and the avenger

Proposal:

• Introduce additional grading states such as published. Avengers must explicitly publish the grading in order for students to view it
• During unsubmission, the submission (if already published), will be 'unpublished'
• Alternatively, we prevent avengers from unsubmitting a submission if it is already published (this means the avenger can only unsubmit in the midst of grading)
• This 'publish' design is similar to the way Coursemology handles grading

More Context: On the submission side, there are currently the following states: attempting, attempted and submitted in the database

On the grading side, there are currently the following states: graded, grading, excluded and none. This is currently computed on the frontend under the function computeGradingStatus in RequestsSaga.ts

See: #2189 Might be relevant: #738, #746, #799