Omniauth auto link LDAP user falls back to find by DN when user cannot be found by uid

Unfortunately, SAML IDs can be an LDAP UID, DN, or something else entirely. UID and DN are most common, though. This adds a fallback scenario so we first try to find a matching LDAP user by UID, then by DN. This will fix a problem for the customer in https://gitlab.zendesk.com/agent/tickets/43298

See merge request !7002