Implement notifications

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`
* `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.

Start by reviewing the existing User, Resource, and Action concepts, along with the page-load WebSocket connection and database integration. Define what must be persisted, delivered to connected users, and fetched when a connection opens; the issue is done when those notification flows and future multi-user delivery are supported.

Written by the indexing model from the issue text.