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/factories/upload.rb

@@ -17,7 +17,7 @@ FactoryBot.define do
       status { "completed" }
       source { nil }
       media_asset_count { 1 }
-      file { Rack::Test::UploadedFile.new("#{Rails.root}/test/files/test.jpg") }
+      files { { "0" => Rack::Test::UploadedFile.new("#{Rails.root}/test/files/test.jpg") } }
 
       upload_media_assets do
         [build(:upload_media_asset, media_asset: build(:media_asset, file: "test/files/test.jpg"), source_url: "file://test.jpg", status: "active")]