Support for release notes and changelogs per version

For a given version of a package, I'd like to be able to display the relevant section of the changelog or release notes.

There's a few moving parts here:
- [x] fetching changelogs from tarballs can be done in https://github.com/ecosyste-ms/archives
- [x] parsing the changelog file to slice it into versioned chunks
- [ ] fetching release notes is already available in https://github.com/ecosyste-ms/repos/
- [ ] storing the chunks against each version in https://github.com/ecosyste-ms/packages
- [ ] optional extra: also doing it for all tags in https://github.com/ecosyste-ms/repos/

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