Low disk space notification

[pludov]

Add a notification for low disk space condition.

The notification must appear in-app then in system notification. This is a
permanent notification (expected to stay until acked by user, and can disappear by itself)

There must be a place where the thresold is set (a configuration page). click on
the notification should display / open this page.

The config page must display the available disk space on storage partition.