Allow creating nested group via UI

What does this MR do?

1. Allow creating nested groups from GitLab UI
2. Add separate page for nested groups of the group. Add filter by name for nested groups
3. Squash All Projects and Shared Projects tabs on group page into one Projects and add filter instead

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

no

Why was this MR needed?

So users can use nested groups feature

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated will be there https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9061
• API support added => API support will be introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9034. This one about GitLab UI only.
• 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