Implement notifications API and update, storage notifications
This mostly implements the notification API as described in #867 (closed) except:
- Showing notifications in app page (I am not sure any more that if should be done).
- Showing centralized notification log.
It also implements the notification UI as described in #867 (closed) (in contrast to ideas presented #1042 (closed)).
It satisfies all the requirements set for the Announcements module as described in #1637 (closed) except:
- Showing a list of all previous announcements.
- Announcements are simply notifications. Notifications on upgrade of FreedomBox can be shown as demonstrated by two notification currently implemented.
It implements the following two notifications:
- Automatic notification on update of FreedomBox to newer version.
- Low disk space warning notification.