Cache Namespace#full_name and Namespace#full_path methods

What does this MR do?

Cache Namespace#full_name and Namespace#full_path methods output

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

no

Why was this MR needed?

To avoid extra sql queries (especially for nested groups). We show full path and full name in many places from UI to API.

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