notification: Change issue assignee notification scope
notification: Change issue assignee notification scope

Now, when issue assignee is changed,
only following people will be received notification.

- Issue author
- Previous & new assignee
- Issue sharers
- Issue watchers

doortts/fork-yobi,doortts/fork-yobi,doortts/fork-yobi,yona-projects/yona,yona-projects/yona,yona-projects/yona,yona-projects/yona,doortts/fork-yobi