get active (visible) courses only

[Ferenc-Frank-Lengyel]

this is a possible solution to resolve issue #772

[danmarsden]

That looks like it may hide the tab for all users - and not just the student themselves?

I think there is a case where the teacher should still be able to see the report for courses that are hidden so we might still need to retain that access somehow.

I think we should check inside attendance_get_user_courses_attendances to see if the currently logged in user has the viewhiddencouress capabiliity and if so, get all users courses, and if not - get only the "active" courses.