Backport "Automatically link Kerberos users and LDAP people" changes to CE

What does this MR do?

Backports changes made in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2405

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

Why was this MR needed?

Reduce CE and EE conflicts

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?