Improve unassigning TA performance in spreadsheet

The performance can be improved by doing a bulk delete_all operation. To
achieve this in SQL using an ANSI-compliant way, a list of IDs of the
association model GradeEntryStudentTa is needed. Therefore, the IDs are
both added to the database and the view.

Running `rake db:migrate` is required for this change.

Tested:
- rspec
- rake test:units
- rake test:functionals