Create subgroups if they don't exist while importing projects

Adds support to create namespaces if they do not exist, together with reuse existing ones.

Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10308

Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10344

• Changelog entry added, if necessary
• 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