EditPost page: categories list is not showed as a tree

[antonrinas]

STEPS TO REPRODUCE:

1. Log in as administrator.
2. Go to Posts->Categories.
3. Add category: Name - "Test category", Slug - "test-category".
4. Add category: Name - "Test subcategory", Slug - "test-subcategory", Parent - "Test category".
5. Go to AllPosts page.
6. Add new post: Title - "Test post for subcategory", assign it to "Test subcategory" (see attachment 1).
7. Go to AllPosts page.
8. Click "Edit" on created post.

EXPECTED RESULT:
categories list is showed as a tree. User may see categories structure.
ACTUAL RESULT:
categories list is not showed as a tree. User cannot see categories structure (see attachment 2).

[ocean90]

Duplicate of #10982.

See also #20054 and [20710].