/user/:user_id/courses should return a course's visible status as well

[tleilax]

Originally, the route was supposed to be used to retrieve the courses of the user calling the API. Since the courses of an arbitrary user can be loaded by this route as well, the visible status of a course should be returned as well. This is useful for a short summary display of teachers ("Visitenkarte") where subsequential calls to the API would be neccessary for each course returned by the route /user/:user_id/courses.