[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/Util/Common.php

@@ -94,6 +94,11 @@ abstract class Common
         return self::ensureLoggedIn()->getNickname();
     }
 
+    public static function userId(): ?string
+    {
+        return self::ensureLoggedIn()->getId();
+    }
+
     public static function ensureLoggedIn(): LocalUser
     {
         if (($user = self::user()) == null) {