allow group leader to try to change permissions
add some more generics
ensure that changed group object has members loaded
fix group leader Javadoc / usage
show permissions downgrade warning for non-system members
clarify meaning of permissions downgrade yes/no buttons
Merge pull request #2752 from mtbc/trac-12362-group-owner-change-permissions

fix #12362, #12396: allow group owner to change permission level (rebased from dev_5_0)