Icon of disabled menu option should be disabled as well

In https://issues.onehippo.com/browse/CHANNELMGR-1415 it was fixed that the publish and discard options are disabled when they are not relevant. However, while the text of the menu option is now disabled, the appearance of the icons for 'Publish' / 'Submit changes' (cka 'Confirm') and 'Delete'  does not change.

When the menu item is disabled, the transparancy of the icons should be 38%.