[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.

plugins/ActivityPub/Util/Model/ActivityCreate.php

@@ -94,7 +94,6 @@ class ActivityCreate extends Activity
             'modified'     => new DateTime(),
         ]);
         DB::persist($ap_act);
-        $ap_act->setObjectMentionIds($note->_object_mentions_ids);
         return $ap_act;
     }
 }