[COMPONENT][Notification] Make logic more generic and robust

Fixed various bugs

Some important concepts to bear in mind:

* Notification: Associated with activities, won't be reconstructed
together with objects, can be thought of as transient

* Attention: Associated with objects, will be reconstructed with them, can
be thought as persistent

* Notifications and Attentions have no direct implications.

* Mentions are a specific form of attentions in notes, leads to the creation of Attentions.

Finally,

Potential PHP issue detected and reported: https://github.com/php/php-src/issues/8199
`static::method()` from a non static context (such as a class method) calls `__call`, rather than
the expected `__callStatic`. Can be fixed by using `(static fn() => static::method())()`, but the
usage of the magic method is strictly unnecessary in this case.

components/Notification/Entity/Notification.php

@@ -28,17 +28,17 @@ use App\Entity\Actor;
 use DateTimeInterface;
 
 /**
- * Entity for attentions
+ * Entity for Notifications
+ *
+ * A Notification when isolated is a form of transient notification.
+ * When together with a persistent form of notification such as attentions or mentions,
+ * it records that the target was notified - which avoids re-notifying upon objects reconstructions.
  *
  * @category  DB
  * @package   GNUsocial
  *
- * @author    Zach Copley <zach@status.net>
- * @copyright 2010 StatusNet Inc.
- * @author    Mikael Nordfeldth <mmn@hethane.se>
- * @copyright 2009-2014 Free Software Foundation, Inc http://www.fsf.org
- * @author    Hugo Sales <hugo@hsal.es>
- * @copyright 2020-2021 Free Software Foundation, Inc http://www.fsf.org
+ * @author    Diogo Peralta Cordeiro <@diogo.site>
+ * @copyright 2021-2022 Free Software Foundation, Inc http://www.fsf.org
  * @license   https://www.gnu.org/licenses/agpl.html GNU AGPL v3 or later
  */
 class Notification extends Entity