Miscellaneous improvements to the nested groups feature

What does this MR do?

• Add nested groups support to the Groups::CreateService
• Show nested groups tab on group page

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

Part of nested groups feature

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/2772, https://gitlab.com/gitlab-org/gitlab-ce/issues/25426