We allow users to configure LDAP attribute preferences. For example,
email can be configured to use `mail`, `email` and `userPrincipalName`,
falling through to the next until a value is found. Prior to this
change, Gitlab::LDAP::Person did not honor this configuration. Now,
the class will honor `name` and `mail` configuration. It does not
handle `username`, or fallback to `first_name` + `last_name` in the
absence of `name`.
