Add the ability to have custom preferences for notifications

Currently, the preferences mechanism is very simple: an event type and either or not the user want to show the events of that type.

Now, we need to have more precise preferences.

Example: see "update" events but only on space A, see "blog post published" event but only for blog B, etc...

We need a custom storage for this preference + a way to customize the query that look for the events + a custom filter on the events returned by the query + a way to inject custom UI components to handle the preferences on the user preferences page.