Refactor attachments to POST directly to a signed upload URL

[alexdlaird]

Instead of POSTing directly to a web node, which subsequently uploads the file to S3, the frontend should first make a lightweight call to the web node asking for a signed upload URL. It should then POST to this upload URL instead, which in prod-like cases would allow uploading of files directly to S3, easing the bandwidth and stress on web nodes.

• Endpoint
• UI to fetch signed URL from new endpoint, then POST there instead

[alexdlaird]

Closing, as the Helium project is not currently active.