Add group members counting related data on namespaces API

This MR adds the information of members_count and parent_id on Namespace::Entity in order to being consumed by GET /namespaces endpoint. This data will be primarily used by customers.gitlab.com.

Note that using GET /namespaces instead GET /groups endpoint give us the ability of having User#namespace and Groups information with a single request.

Backport MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2284

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
• 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-com/customers-gitlab-com/issues/122 and https://gitlab.com/gitlab-com/customers-gitlab-com/issues/123.