fix sorting for grading

[thortol]

Description

Currently there is a bug in source academy grading table where users are unable to sort by username and group

Changes

• There was a bug where a left join condition was done on user.id == submissions.student_id and user.id == courseRegistration.user_id
• However, in the schema the conditions are courseRegistration.id == submissions.student_id and courseRegistration.user_id == user.id
• An additional bug was found during testing where the same row kept appearing every page. This happens when submission.inserted_at is the same which prevents LIMIT from working as the sort may be inconsistent.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Code quality improvements

Checklist

• I have tested this code
• I have updated the documentation

[coveralls]

coverage: 94.38% (+0.002%) from 94.378%
when pulling f84156e on thortol:fix-sorting-for-grading
into ddd4378 on source-academy:master.

[RichDom2185]

LGTM, thanks!