Resolve "Pull repository mirroring: Support for SSH keys"

What does this MR do?

Introduces SSH public-key authentication to pull mirroring.

Tasks:

• Configuration backend (username/password auth)
• Configuration backend (public-key auth)
• Configuration frontend (wireframe)
• Configuration frontend (complete) https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2550
• SSH known_hosts file generator (backend)
• SSH host key fingerprint verification (backend)
• SSH host key fingerprint verification (frontend) https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2550
• gitlab-shell.fetch_remote updated to support pubkey auth
• gitlab-shell#95 (closed)
• Comprehensive test coverage. Pull mirroring seems to be mostly untested at the moment
• Documentation!
• Backports to CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13275

Are there points in the code the reviewer needs to double check?

The import_url and project_import_data data is shared with CE, so we need to pay attention to backports when refactoring.

Why was this MR needed?

Scheduled feature for %9.5

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?

Closes #98 (closed) https://gitlab.com/gitlab-org/gitlab-ce/issues/21391