Avoid n+1 queries in marks_graders view

This mainly improves the performance of the index page of assigning graders to spreadsheet students. There are also some minor refactoring.

Tested:
- rspec
- rake test:units
- rakte test:functionals
- local server
Unused method argument - `grade_entry_form`. If it's necessary, use `_` or `_grade_entry_form` as an argument name to indicate that it won't be used. You can also write as `students_with_assoc(*)` if you want the method to accept any arguments but don't care about them.