add core.group_add_user_after event for group_user_add()

group_user_del() already has 'core.group_delete_user_before' event, though it has invalid functionality and should break removal process if any variable is changed to false by an extension. So extensions can deny to add a user to group.

And logically we need 'core.group_delete_user_after' event for sure.

But the main problem is that adding users to groups is not synced with extensions, so we need at least 'core.group_add_user_after' event triggered in group_user_add(), and for sure would be perfect to have 'core.group_add_user_before' which allows extensions to block adding users to a group.