Make activity Hide/show option compatible/synchronize with course index

As a teacher
I want to be able to hide and show activities and see the change applied in the course index
so I don't need to reload the page to confirm the changes were properly applied.

Acceptance criteria:

1. Hidden activities are styled differently in the course index
2. Activity visibility is synchronized between Course index and course main content area.
3. When an activity is shown in the course main content area is also marked as visible in the course index.
4. When an activity is  hidden in the course main content area is also marked as hidden in the course index.