Add nested groups support to the routing

What does this MR do?

It allows routing with / in namespace name

Why was this MR needed?

For nested groups feature(https://gitlab.com/gitlab-org/gitlab-ce/issues/2772). We need URI like /group/subgroup/project be routed correctly

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