refactor: Refactor grading related functions to use atoms

[GabrielCWT]

Changes

• Implement helper function to parse string to atom and from string to int.
• Refactor submissions_by_grader_for_index to have its controller parse the inputs into atoms and pass an atom hashmap to the function instead.
• Refactor sorting functions for submissions_by_grader_for_index to use atoms and use pattern matching.
• Combine sorting functions into one, using sort_direction to order the rows either by asc or desc
• Update corresponding tests

Closes: https://github.com/source-academy/backend/issues/1174 https://github.com/source-academy/backend/issues/1175

[coveralls]

coverage: 94.324% (-0.05%) from 94.372% when pulling 892b5d434843834bc4ec08a1043486455544120f on GabrielCWT:refactor/string-to-atoms into c0d95e9df86f513b181d8604da37245e6cba8cc3 on source-academy:master.