[WIP]Add notification when story, comic, wallpaper or urban legend is posted.

[#1216](https://github.com/fjordllc/kowabana/issues/1216), [#1302](https://github.com/fjordllc/kowabana/pull/1302)に関連
怖話に新規投稿があった時に、slackに通知を流す。