doi: Issue DOIs

[kousu]

GIN can issue DOIs; I assume that only works on their flagship server, not on self-hosted servers, because becoming a DOI issuer is expensive.

But it would be great if we could provide this for datasets we host; either:

• integrate the expensive official DOI issuance API
• maybe CrossRef's DOI Service can bridge the gap, for cheaper

[kousu]

I realized that GIN doesn't actually issue DOIs in their Gogs/Gitea fork. They have a separate web app: https://doi.gin.g-node.org/. The only patch they've made to this is to add a menu link to it:

https://github.com/G-Node/gogs/blob/2efac57ed554192eea15f1061d2a08c36c38a683/templates/explore/navbar.tmpl#L7

If we also do DOIs, I think this is the way to go. Maybe we can get their DOI app from them? It seems to be here: https://github.com/G-Node/gin-doi