fix saml login for good

[butonic]

if cn or uid aru used in the saml token, the ldap app will not use them, because core currently has to use the getUsers method, which does a generic user search ... and also appends a wildcard, which is why using the email in the token is recommended to prevent ambiguous logins where more than one user matches the filter.

The proper solution in core is to introduce an api that tries to lookup a single user.

A quickfix in user_ldap would be to add the login attribute to the search filter in https://github.com/owncloud/user_ldap/blob/master/lib/User/Manager.php#L552-L556

[PVince81]

old ticket, no one actively working -> backlog

[butonic]

related https://github.com/owncloud/user_ldap/pull/238 and https://github.com/owncloud/core/issues/29503#issuecomment-462800169