Persistent notifications

closes #3057
- add Notification model
- update injected Notifications object to handle persistent notifications
- load server notifications on setup if logged in otherwise on successful sign-in
- changed all notifications.closeAll calls to closePassive
- close all notifications on sign out

Known issues:
- Needs tests
- The check for a logged in user in the 'loadServerNotifications' action will need updating once #3060 lands
It is from the server side - `location` and `dismissible` will be used to place notifications at different places on the screen, `dismissible` is self-explaining.

@kevinansfield While you’re at it - can you fix that `dismissable` (sic!) in https://github.com/TryGhost/Ghost/blob/master/core/server/api/notifications.js#L88 and in the tests?