Decouple student rights from student role
PhD students can sign up for master courses in some situations. Currently this is a problem, because enrolling is not possible in project forum and they also don't get any of the intended permissions within courses. Even if they have those permissions, a lot of the required buttons do not show up for them.
We should refactor the permissions in such a way that student-related course specific rights are given based on enrolment rather than global role (student/employee). Additionally, some buttons should be shown depending on permission (i.e. enrolment) rather than on "is this user a student".
Staff members should not have an enrolment button but it should be possible to select any user when creating an enrolment in the management panel.
We should make some tests to ensure we don't have regressions.