Add support for nested groups to admin routing

What does this MR do?

Changes admin routing to work with wildcard namespace id. Since rails resources does not work well with / inside *id I need to replace resource with scope + match combo. As result some routing naming changed because resource put action names as prefix to the path while match always does suffix. As bonus I replaced admin_namespaces_projects_path with admin_projects_path.

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

no

Why was this MR needed?

To make nested groups work in admin area

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