Prevent inaccurate feedback notifications where grade item hidden

If the grade column for a mod_assign assignment is hidden and a grader provides feedback, the student is sent a message saying something like "[teacher] has posted some feedback on your assignment submission for [assignment name]. You can see it appended to your assignment submission."

This is confusing for the student as they can't actually see it until the grader has made the grade column visible (which, in our institution at least, can be well over a week after the feedback is given).

To replicate issue:

1. Create an assignment
2. Set feedback comments and feedback files to yes
3. Log in as a student and make a submission
4. Log back in as a teacher and hide the gradebook item for the assignment
5. Give feedback on the item
6. The student will get a notification message saying "[teacher] has posted some feedback on your assignment submission for [assignment name]. You can see it appended to your assignment submission."
7. Log in as the student and follow the link in the email. You don't see the feedback.