Nested groups feature improvemetns

What does this MR do?

• Refactor and rename some methods for Namespace class
• Adds new methods to user model
• Includes nested projects in RefreshAuthorizedProjectsService

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

Why was this MR needed?

Make code better and integrate nested groups more deeply

Screenshots (if relevant)

no

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