Improve performance of assigning TAs to students

This uses the same technique as that for the performance of assigning graders to groupings and criteria but slightly different in implementation as GradeEntryStudent might not exist at the time of assigning TAs to GradeEntryStudent; they need to be created on the fly.

Tested:
- rspec
- rake test:units
- rake test:functionals
Shadowing outer local variable - `student_ids`.