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]`.

test/test_helpers/upload_test_helper.rb

@@ -7,7 +7,7 @@ module UploadTestHelper
       source = { source: source_or_file_path }
     else
       file = Rack::Test::UploadedFile.new(Rails.root.join(source_or_file_path))
-      source = { file: file }
+      source = { files: { "0" => file } }
     end
 
     perform_enqueued_jobs do