FEATURE: Support private attachments when using S3 storage (#7677)

* Support private uploads in S3
* Use localStore for local avatars
* Add job to update private upload ACL on S3
* Test multisite paths
* update ACL for private uploads in migrate_to_s3 task

lib/tasks/uploads.rake

@@ -425,6 +425,10 @@ def migrate_to_s3
         options[:content_disposition] =
           %Q{attachment; filename="#{upload.original_filename}"}
       end
+
+      if upload&.private?
+        options[:acl] = "private"
+      end
     end
 
     etag ||= Digest::MD5.file(path).hexdigest