[Work-in-Progress] Correct answers and points (#431)

[Deep-Chill]

• Added preliminary CorrectAnswer model
• Added preliminary scoring logic in AssessmentCompleteAPIView

Need clarifications on :

• If we should add a field for the correct answer in the CorrectAnswer model or use the one within the Question model
• How exactly answer.measured relates to the Choice object that is the correct answer? Meaning if we have Answer.measured, what exactly do we check to ensure it is the correct answer?
• Where should we return the score for the correct answers, if we should return it in the API View at all?

[Deep-Chill]

Superseded by https://github.com/djaodjin/djaopsp/pull/445