Allow manual reference papers to be added

[ewan-escience]

For software, reference papers cannot currently be added manually (only through DOI search). We should also allow reference papers with an OpenAlex ID to be added (their citations can be harvested since #1306), and even reference papers without a DOI or OpenAlex ID. We might not be able to harvest citations of such reference papers, but they might still exist and should be displayed as such.