[ATTACHMENTS] Further refactoring

Some key points:
- Components and Plugins shouldn't extend Module directly
- Avatars should be fetched via GSActor ID, not by nickname as that isn't unique
- Avatar now is a separate Component
- Common file utilities are now to be placed in Core\GSFile, this will
  handle storage and trigger validation
- Some bug fixes

src/Entity/Note.php

@@ -196,7 +196,7 @@ class Note extends Entity
     public function getAvatarUrl()
     {
         $url = null;
-        Event::handle('GetAvatarUrl', [$this->getActorNickname(), &$url]);
+        Event::handle('GetAvatarUrl', [$this->getGSActorId(), &$url]);
         return $url;
     }
     public static function getAllNotes(int $noteScope): array