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

added 1 commit

• fe473039 - Show nested groups tab on group page

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marked the task Added for this feature/bug as completed

marked the task All builds are passing as completed

marked the task Conform by the style guides as completed

marked the task Branch has no merge conflicts with master (if it does - rebase it please) as completed

marked the task Squashed related commits together as completed

added 1 commit

• 10de4e3b - Show nested groups tab on group page

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marked the task Changelog entry added as completed

marked the task Conform by the merge request performance guides as completed

Needs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8071 to be merged first

@DouweM please review

assigned to @DouweM

assigned to @dzaporozhets

added 1 commit

• 283e868e - Refactor nested group related code

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added 1 commit

• 283e868e - Refactor nested group related code

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resolved all discussions

enabled an automatic merge when the pipeline for 283e868e succeeds

merged

mentioned in commit 23fc1f66

mentioned in issue #2772 (closed)