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
- fixed dismissable/dismissible spelling in server API & tests
- add notifications.closeNotification method so DELETE calls can be made for server-originating notifications

Known issues:
- Needs tests