Add ldap_access_filter option
    
    This option (applicable to access_provider=ldap) allows the admin
    to set an additional LDAP search filter that must match in order
    for a user to be granted access to the system.
    
    Common examples for this would be limiting access to users by in a
    particular group, for example:
    ldap_access_filter = memberOf=cn=access_group,ou=Groups,dc=example,dc=com