Allow admins to make system wide announcement 📢

![lol](http://37.media.tumblr.com/tumblr_ly0g2psDOS1r556mmo1_250.gif)
For #70.
The iteration itself should be in background. the simplest solution is use a separate thread, but then what if it stops half way ? The proper way will be create a background task to queue the emails, and this task should be able to handle failure (since system wide means potentially it could be thousands of emails)