Ticket 50077 - RFE - improve automember plugin to work with modify ops
    
    Description:
    
    Previously automember was only invoked for ADD operations.  This enhancement
    allows it to work with modify operations, and it will also maintain the
    correct memberships.  So if a modify changes which groups the user would
    belong to, it will add the user to the new group, and remove them from the
    old group.
    
    https://pagure.io/389-ds-base/issue/50077
    
    Reviewed by: spichugi & firstyear (Thanks!!)
    
    (cherry picked from commit 7096094e0f26ad3f4acbbffeefc0a174a791686d)