Add 'Draft' filter to assignment grading table

Manual testing (this is optional as behat test does the same) Create a course. Enroll two users with a student role. Create an assignment activity with  Require students to click the submit button set to Yes . Login as User1. Add submission and submit the submission. Login as User2. Add submission and leave it as draft (don't submit). Login as an admin/teacher. View all submissions. Choose the "Submitted" filter. Confirm , that only User1's submission is listed. Choose the "Draft" filter. Confirm , that only User2's submission is listed. Clear the filter. Confirm , that both submissions are listed. Click Grade button for User1 submission in order to navigate to the grader. Click on the filter icon and select the "Draft" filter. Confirm , that User2's submission is displayed.