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

[josh1248]

Relevant PRs: Frontend: https://github.com/source-academy/frontend/pull/2787 Backend: https://github.com/source-academy/backend/pull/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.