Allow to manually add members to a LDAP-synced group
Currently, it's possible to override LDAP-synced member permissions but it's not possible to add members manually without them being removed on the next LDAP-sync for the group.
We should allow to add members manually and they shouldn't be wiped out from the group automatically. That shouldn't be too hard to do since LDAP-synced members have a ldap
flag, and other members don't. Access requests should also work normally.
The feature proposal comes from https://gitlab.com/gitlab-org/gitlab-ee/issues/1702#note_23399260.