Identify user profile by stored "dn" attribute if finding it via "uid" fails

As discussed in this forum thread:

https://forum.xwiki.org/t/ldap-authenticator-identify-user-profile-page-by-dn-to-prevent-profile-duplication/7806/2

if the XWiki user profile data cannot be found from the given login data in the uid field of the XWiki.LDAPProfileClass, check later if it can be identified via the dn.

This helps if the attribute used to log in is changed when LDAP related user profiles already exist.