Show a "non active" message to mock opening

Since "[mock opening](https://codingcoach.io/mock-openings)" is no longer active we should show e message that this modules it not active. If someone is willing to lead this, we can consider bringing it back.

Start at the mock opening module linked in the issue and inspect its current inactive behavior. Show a clear message that the module is not active; the work is done when visitors to the mock openings page see that message instead of the inactive module.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.