uploads: allow uploading multiple files from your computer at once.

Allow uploading multiple files from your computer at once.

The maximum limit is 100 files at once. There is still a 50MB size limit
that applies to the whole upload. This limit is at the Nginx level.

The upload widget no longer shows a thumbnail preview of the uploaded
file. This is because there isn't room for it in a multi-file upload,
and because the next page will show a preview anyway after the files are
uploaded.

Direct file uploads are processed synchronously, so they may be slow.

API change: the `POST /uploads` endpoint now expects the param to be
`upload[files][]`, not `upload[file]`.

app/policies/upload_policy.rb

@@ -18,6 +18,6 @@ class UploadPolicy < ApplicationPolicy
   end
 
   def permitted_attributes
-    %i[file source referer_url]
+    [:source, :referer_url, files: {}]
   end
 end