Notifications marked as unread disappear if the server stops returning it

[sirbrillig]

Since there's no way to "mark as unread" in the github notifications API (at least, last time I checked), the "mark as unread" behavior in gitnews-menubar is internal to the app's local cache of the data. However, since github thinks the notification is already read, it will eventually be removed from the notifications API response since it only lists recent notifications.

It would be nice if a notification, when marked as unread, would stay around until it was clicked or marked as read. Extending this thought, the notification shouldn't immediately disappear after it has been marked as read either; perhaps we could use an internal expiration system in general.

[sirbrillig]

The persistence was buggy and was reverted, so this is an issue again.

[sirbrillig]

This is related to #154