Add the ability to check logins against the old account.

This means we need to override LDAPAuthentication2's authenticate() method because we want to try
the LDAP login if the local login fails. This is needed where someone has the same username on the
wiki as they do in LDAP. LDAPAuthentication2's authenticate() method will just fail if the username
exists and the password for it doesn't work. Since we're trying to merge the accounts, we don't
want to just fail on the wiki account failure.