mod/assign: Add submission view filters - "has extension" and "submitted late"

Test 1: 1. Create an assignment. Allow 1 week between due date and cutoff date 2. Log in as student 3. Submit assignment after due date, but before cutoff date 4. Log in as teacher 5. View/grade all submissions 6. Apply filter “submitted late” 7. Only student who submitted late should appear 8. Change filter to “all submissions” 9. All students should appear Test 2: 1. Create assignment. Set the due date and cutoff date to be the same 2. Save and display 3. View/grade all submissions 4. Verify “submitted late” does not appear in the filter list Test 3: 1. Create assignment. Uncheck due date and cutoff date 2. Save and display 3. View/grade all submissions 4. Verify “submitted late” does not appear in the filter list