message: now Unit is also visible if the visible flag is undefined (for migration reasons)
diff: @@ -242,7 +242,7 @@ export class CourseController {
  populate: {
  path: 'units',
  virtuals: true,
  - match: {$or: [{visible: !isTeacherOrAdmin},{visible: true}]},
  + match: {$or: [{visible: undefined},{visible: true},{visible: !isTeacherOrAdmin}]},
  populate: {
  path: 'progressData',
  match: {user: {$eq: currentUser._id}}
  