Nested groups improvements pt 2

What does this MR do?

1. Expose full_name and full_path attributes of the Group to API
2. Use full_name and full_path for different dropdown/search autocomplete
3. When search inside group - look for results in nested groups too
4. Update group api doc with some attributes that are already exists but not mention in docs

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

no

Why was this MR needed?

Make nested groups more integrated into GitLab

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