Add papers to RSS feed for links

[capotej]

Read AGENTS.md for information on working with this repository.

Problem

Currently, links/feed uses app/views/links/feed.atom.builder to render an RSS feed of Link model entries.

Let's add support for the newly created Paper model and render those entries as part of the same feed.

Instructions

• Add support for rendering paper entries in the links feed.
• Write a test for links/feed that ensures the current and new behavior for any mix of papers and links.
• Ensure the proper URL is used when rendering entries for arXiv and regular papers.

Outcome

• Visiting links/feed renders an RSS feed including Link and Paper urls.

Environment

If ruby is not available to run tests, you can run tests remotely using GitHub Actions by pushing commits to a branch.