Describe the bug
Assemblies can be set as child of another another assembly. This is not properly checked, so you could set an assembly as a child of one of its own childs. When this happens, the assemblies are not accessible from the assemblies admin panel.

To Reproduce
Steps to reproduce the behavior:

1. Have an Assembly A
2. Have an assembly B
3. Edit Assembly B and set it as child of assembly A. See how assembly B is no longer directly accessible from the admin panel, but instead you have to click a special button to explore the children assemblies. You can access it from that new page.
4. Edit assembly A and set it as child of assembly B. Assembly A can no longer be accessible from the admin panel, and thus assembly B can't be accessed either.

Expected behavior
I should not be able to set an assembly as children of its own childs, to avoid this kind of circles.

Screenshots
None

Stacktrace
None

Extra data (please complete the following information):

• Decidim Version: master