File uploads on objects storage should use hashed storage

What does this MR do?

Make the FileUploader store its file in a project/namespace agnostic way (see HashedStorage). This MR will basically enforce the use of hashed storage path for remote file upload.

Are there points in the code the reviewer needs to double check?

This changes the value of Upload.path for uploads from the FileUploader, but as we are now storing the secret in the model, this will still work and also normalize the Uploads.

Why was this MR needed?

File uploads on object storage would be inaccessible after a project rename.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

This should supersede !4537 (closed) Closes #4952 (closed)