Add sign-in link to registry

[mishaschwartz]

This is a link that is directly to the sign-in page for a given node. This may or may not be distinct from the node's home page.

[fmigneault]

Still in draft, but I find this proposed IANA Link relation types relevant:
https://www.ietf.org/id/draft-ietf-httpapi-authentication-link-01.html

Some items such as register-user seem helpful to better guide with our registration_status field.
The authenticate and logout would be of interest for the mentioned sign-in page.

Weirdly enough, accepted IANA link relation types do not have anything about authentication other than OpenID-specific entries.
https://www.iana.org/assignments/link-relations/link-relations.xhtml
https://openid.net/specs/openid-authentication-2_0.html#rfc.section.7.3.3