Add/hook up the additional endpoints for xp

[indocomsoft]

Backend endpoints to be added/hooked up after source-academy/backend#232 is merged:

• User, add total_xp
• Grading, add xp, xp_bonus, xp_adjustment
• Assessment, add max_xp, xp, grade

[remo5000]

Thanks for the summary of the properties @indocomsoft . I'll take a swing at this today

[remo5000]

Fixed by #334