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.
Fix functional tests for new unassign parameters
Add spec for new unassign function
Merge pull request #1686 from zhangsu/unassign_graders

Improve unassigning TA performance in spreadsheet