Allow login by email through LDAP

Does that make sense @erwint ? I think you could probably just turn this into a generic lookup like this:

```
String cn = env.get("ldap.userAttr") + "=" + sanitizedUser + "," + userDn;
try (LDAPResource ldap = new LDAPResource()) {
    Entry user = ldap.lookup(cn);
    if(user != null){
        user.getDn().getName();
    } else {
        throw new RuntimeException("User not found");
    }
} 
```