add atom feed endpoint

[BasiqueEvangelist]

Adds an atom feed that lists all visible versions for a project.

Current issues:

• Description isn't rendered, and Atom requires there to either be a human-readable summary or content

[BasiqueEvangelist]

How do you run that check locally :thonk:
anyways, that's even more reasons for it to be in draft

[triphora]

I think it would just be better for this to be in the frontend, tbh. https://github.com/jpmonette/feed is quite great and doesn't require anywhere near as much maintenance burden.

[BasiqueEvangelist]

Closing, as the Knossos PR will most likely replace this