Add option not to display grader to students

Testing is easier if you use two browsers: one logged in as a teacher, one logged in as a student.

Note: You'll need to be able to capture and monitor outbound emails - so please use something like mailcatcher and make sure to set up Moodle's SMTP server to point to this.

1. [Teacher] Create an assignment with the 'Hide Grader' option off (it should be off by default).
2. [Student] Add a submission to the assignment
3. [Teacher] Grade the submission, and make sure 'Notify student' is enabled.
4. Run cron:

   php admin/cli/cron.php

5. [Student] The student will receive an email, with the grader's name visible.
6. [Student] Visit the 'View Submission' page. Verify the grader's name is visible on the page.
7. [Teacher] Enable the 'Hide Grader' option in the assignment's settings.
8. [Student] Visit the 'View Submission' page. Verify the grader's name is not shown on the page.
9. [Teacher] Re-grade the assignment, and make sure 'Notify student' is enabled.
10. Run cron:

    php admin/cli/cron.php

11. [Student] The student will receive an email, and the grader's name will not be visible.
12. [Teacher] Edit the assignment's permissions, and give the 'Show Hidden Grader' permission to the Student role.
13. [Student] Visit the 'View Submission' page. Verify the grader's name is now visible.
14. [Teacher] Re-grade the assignment, and make sure 'Notify student' is enabled.
15. Run cron:

    php admin/cli/cron.php

16. [Student] The student will receive an email, with the grader's name visible.