The availability "Make available but not shown on course page" of an activity is not kept, when the visibility of the topic is changed.

This behaviour is a problem because activities unintentionally have their availability changed. An explicitly set availability should be kept when moving the activity.

How to reproduce:

1. In a course, hide a topic.
2. Create an activity (e.g. text page) in this topic.
3. Change availability of the element to "Make available but not shown on course page".
4. Move the activity to a different (visible) topic or change the topic to visible.
5. The activity is now shown on the course page.