Describe the bug

Community admins see the button to create top-level communities, although they shouldn't.
Since they administer one or more communities, they should be able to create sub-communities but not top-level ones.

If they click on the top-level community creation button, they see the form, but an error prevents them from saving it.

To Reproduce

Steps to reproduce the behavior:

1. Log in as Community admin on https://demo.dspace.org/home
2. On the admin bar, click on New -> Community
3. Select Create a new top-level community
4. Fill the form
5. Save
   An error will correctly block the creation

Expected behavior

The Community admins don't see the button for creating top-level communities in the admin bar.