EE port of "backport SSH host key detection code to CE"

What does this MR do?

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22577

This is important preparatory work for the %11.5 deliverable: https://gitlab.com/gitlab-org/gitlab-ce/issues/49565

In addition to moving some code to CE, we also need to modify how the existing pull mirroring frontend uses the SSH host key lookup endpoint. Instead of assuming that the previous host keys are stored in the ProjectImportData model for the project, the frontend passes the current value to the endpoint directly. This is a large urlencoded string, but it shouldn't generally push the URL length over 2048 bytes - it's ~600 bytes on my installation, for instance. Still, I'm open to this changing so we pass the parameters in the request body instead, if necessary.

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/49565

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?