Export CSV function regression after transfer of pagination of grading submissions from frontend to backend

[josh1248]

Relevant PRs:
Frontend: #2787
Backend: source-academy/backend#1065

The backend pagination has diminished the export CSV function, which can now only save the backend entries it gets.

Possible fixes:

• An updated function could take the total number of grading submissions, suppose X, from the backend response. Then, it can submit a query with {offset: 0, pageSize: X} to circumvent the LIMIT implementation in the backend.