Add 'Granted extension' filter to mod_assign grading table

1. Generate a test course with multiple students enrolled.
2. Create an assignment with due date enabled.
3. View all submissions and grant one or more students an extension.
4. Select 'Granted extension' from the 'Filter' select menu.
5. Verify that only the students who were granted an extension are now listed.
6. Clear the filter and verify that all students are displayed again.

Repeat the remaining steps for both 'Clean' and 'Boost' themes:

1. Click the 'Grade' button for one of the students.
2. On the student's grading/annotation page, select the 'Granted extension' filter (click the filter icon under the user selector in the top-right corner to reveal the filters).
3. Verify that the filter works as expected, and switch to another user.
4. Clear the filter and verify that all students are listed in the user selector again.