Github import rake task

Adds a rake task to import Github projects into GitLab. This way we don't have to be dependent on Sidekiq to import large projects.

• 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
• 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?

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/28183