Support course-specific student notes

[troeger]

Idea by @aibo21:

Use cases:

In some courses, it is common to have course-specific groups of students. These groups are defined by the tutors, so this data is not being maintained by students (in comparison to something like the student ID).

Tutors sometimes want to leave internal comments about students (regular cheater, special grading treatment needed, ...) that are visible regardless of the particular submission.

The course-specific student notes must be visible in the submission overview and the grading table. It remains unclear at them moment where they are editable.