Apply ldapUserFilter on members of group

Refers to issue #8220

user_ldap configured with custom filters for active directory access
(group-member-association is "member"). Then it can happen that the
members of a group contain members that don't belong to the users
available in Nextcloud (the most trivial reason is that the user filter
contains "(!(UserAccountControl:1.2.840.113556.1.4.803:=2))" to exclude
disabled users from being imported).

This can be fixed by applying the ldapUserFilter when resolving the UID
for a DN fetched from the group's member list.

Signed-off-by: Roland Tapken <roland@bitarbeiter.net>