Hide children section for milestones

[oliverguenther]

https://community.openproject.org/work_packages/54983

[cbliard]

I have only reviewed behavior and not the code yet.

It works properly in these scenarios:

• Selecting a milestone wp and going to relations tab, the children section is not visible
• Changing a wp type from milestone to task, the children section becomes visible

It does not work properly in these scenarios:

• Going to a Task wp, the children section of the relations is not visible. It should be visible.
  • Initially I saw it while reloading but it seems to alway be hidden as it also happens when navigating from a wp list view.
• Changing wp type from Milestone to Task shows the children section, but then changing it to Milestone again does not hide it. It should become hidden.

That's probably why some tests are failing.