R1:b0802a779762

Resolve LDAP registration DAO exception due to empty username

Summary:
When logging in as an LDAP user for the first time (thus registering), a DAO exception was being thrown because PhabricatorLDAPRegistrationController wasn't passing in a username to PhabricatorUser::setUsername().

Somewhat separately, since either the PHP LDAP extension's underlying library or Active Directory are returning attributes with lowercased key names, I have to search on sAMAccountName and look for the key samaccountname in…