More powerful/extensible notifications

- Notifications can be delivered in multiple ways (alert, email)
- Different notification types can implement interfaces to allow
themselves to be delivered in these various ways
- User preferences for each notification type/method combination are
automatically registered

src/Core/Models/User.php

@@ -356,7 +356,7 @@ class User extends Model
             $defaults[$k] = $v['default'];
         }
 
-        return array_merge($defaults, (array) json_decode($value, true));
+        return array_merge($defaults, array_only((array) json_decode($value, true), array_keys(static::$preferences)));
     }
 
     public function setPreferencesAttribute($value)
@@ -372,6 +372,16 @@ class User extends Model
         ];
     }
 
+    public static function notificationPreferenceKey($type, $sender)
+    {
+        return 'notify_'.$type.'_'.$sender;
+    }
+
+    public function shouldNotify($type, $method)
+    {
+        return $this->preference(static::notificationPreferenceKey($type, $method));
+    }
+
     public function preference($key, $default = null)
     {
         return array_get($this->preferences, $key, $default);