Modify namespace name and path validation

What does this MR do?

• Allow same namespace name with different parent_id
• Allow same namespace path. Uniq validation should be handled by routes table

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

migrations

Why was this MR needed?

So we can use same name in different nesting like:

gitlab-ce/foo
gitlab-com/foo
foo/gitlab-ce

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