Frontend support SSH public-key authentication for pull mirroring

What does this MR do?

Adds frontend part for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2551

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [ ] 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?

#98 (closed)