[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

plugins/Cover/Controller/Cover.php

@@ -29,8 +29,8 @@ use App\Util\Common;
 use App\Util\Exception\ClientException;
 use App\Util\Exception\RedirectException;
 use App\Util\Exception\ServerException;
-use Component\Media\Media;
-use Component\Media\Media as M;
+use Component\Media\Attachment;
+use Component\Media\Attachment as M;
 use Symfony\Component\Form\Extension\Core\Type\FileType;
 use Symfony\Component\Form\Extension\Core\Type\HiddenType;
 use Symfony\Component\Form\Extension\Core\Type\SubmitType;
@@ -97,7 +97,7 @@ class Cover
             if (explode('/',$sfile->getMimeType())[0] != 'image') {
                 throw new ServerException('Invalid file type');
             }
-            $file     = Media::validateAndStoreFile($sfile, Common::config('cover', 'dir'), $title = null, $is_local = true, $use_unique = $actor_id);
+            $file     = Attachment::validateAndStoreFile($sfile, Common::config('cover', 'dir'), $title = null, $is_local = true, $use_unique = $actor_id);
             $old_file = null;
             $cover    = DB::find('cover', ['gsactor_id' => $actor_id]);
             // Must get old id before inserting another one