Can we make the Trix editor upload attachments directly to S3/Cloudinary

[jagthedrummer]

Currently when you add a file or an image inside the Trix editor it uploads the file to the application server, which then uploads it to the storage service. We already have it setup so that file_fields will upload things directly to the storage service. It would be nice if the Trix editor did the same.

Here's a gist where someone has configured Trix to use direct uploads to S3.

https://gist.github.com/TheRealNeil/2ba115b8b62f37b591dddbc8f278f3bb

Maybe we can do something similar, and modify it to be smart about Cloudinary uploads as well. (Cloudinary bits are related to: #1617)