Add image upload and download from media url

[sampaccoud]

Purpose

We want to be able to attach images to a document. Strict access control should be applied on all images of a document.

Proposal

• upload images to a folder next to each document's content file : `{document.id}/attachments/{image_filename}
• allow downloading attachment images from object storage via the nginx ingress. Access control is done thanks to nginx subrequests.
• Update the helm chart to add the new media url with access control delegated to Django