Implement notifications

[Benjozork]

Self-explanatory. Could work via a WebSocket connection established at page load.

A possible architecture would be this :

• Notification object, source: NotificationSource, target: User / targets: Set<User>
• NotificationSource interface for Resource objects that can fire notifications, i.e. articles, comments ...
• notify() function on User that stores the notification in DB and finds an open WS connection to notify connected clients for the particular user(s) (notifications sent to multiple users might exist in the future)
• Fetch all notifications from DB on WS open ...

WIP. More details to be added, esp. in regard to supporting Actions i.e. REPLY that would fire notifications automatically when the Actions framework is implemented.

[Benjozork]

Somewhat stable in 078b6b0. Needs polish.