Fixes two issues found with our LDAP handling code. First, we now follow referrals in both LDAP calls, as some LDAP systems will return a referral instead of the original record. Second, we now make sure to handle multiple search result pairs properly by further filtering based on the presence of the 'mail' attribute when we have multiple valid pairs. This CL also adds tests for all of the above cases.
Currently, we use the Quay username via `verify_user` when we go to create the encrypted password. This is only correct if Quay has not generated its own different username for the LDAP user, and fails if it has. We therefore add a new method `confirm_existing_user`, which looks up the federated login for the LDAP user and then runs the auth flow using that username.
