Support assignment feeback

[troeger]

Students have no way to give feedback about an assignment, especially when their validation fails. One proposal:

• Assignments get a new author field.
• It gets automatically the current, assignment-creating, user as value.
• The student details view gets a text field that allows to send feedback, without revealing the authors identity.
• Feedbacks are stored in the database and can be scanned per assignment by all people with backend rights.

[troeger]

Check this: https://djangopackages.org/grids/g/feedback/