GitHub classroom: use openAt date in course-info.json for assessment display in front-end

From GitHubClassroom.tsx

type GitHubAssessment = {
  id: string;
  title: string;  
  openAt: string;  
  closeAt: string;  
  published: string;  
  coverImage: string;  
  shortSummary: string;  
  acceptLink: string;  
  repoPrefix: string;  
};  

[ChengGeng97]

More details:
The GitHubClassroom page should make use of the openDate when drawing cards. If the openDate has not yet been reached, the button to Accept/Open/Review Answer should not be shown.

After this is implemented, we can consider providing a better sorting algorithm between 3 types of assessments: Upcoming, Open and Closed.

Assessments should be sorted in the order of Upcoming, Open and then Closed.

Amongst Open assessments, the assessments should also be sorted such that assessments with nearer closeAt dates are nearer to the top.