[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/Core/Modules/Plugin.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace App\Core\Modules;
+
+abstract class Plugin extends Module
+{
+}