Automatically link Kerberos users and LDAP people

What does this MR do?

Automatically links users who log in via Kerberos to their LDAP person

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

There are a number of caveats on the operation of this feature. In particular, the Kerberos realm must match the domain part of the LDAP user's distinguished name. This is documented, but I'm not clear on what percentage of installations meet this requirement.

We could make the mechanism configurable, and allow users to specify a custom LDAP field to look up the Kerberos principal from, but should this come as a follow-up MR in %9.5 or can it be left until later?

Why was this MR needed?

Much-requested customer feature

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 #1552 (closed)