Implement group_filter for LDAP integration

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Zendesk issue: https://gitlab.zendesk.com/agent/tickets/16230

In some LDAP systems users and groups appear within the same OU structure. In this case, when configuring a group_base and trying to link LDAP groups in the UI, both users and groups show in the autocomplete field. Supporting a group_filter option in configuration would allow administrators to set a filter such as (objectClass=group)

cc/ @JobV I don't think this would be too complicated. Please schedule if you think it sounds like a good idea.