Add `solid:oidcIssuer` link in profile template

https://github.com/solid/node-solid-server/blob/master/default-templates/new-account/profile/card%24.ttl
has no triple for solid:oidcIssuer.

This is because we expect the client to optimistically assume that if https://xyz.abc/foo/bar#me describes a human being, then https://xyz.abc may be usable as a trusted OIDC issuer for webid-oidc for that person.

But then the client has to retrieve https://xyz.abc/.well-known/openid-configuration, see if all the right token types, flows, etcetera are announced, and that requires a simple addressbook app to know a lot about how webid-oidc works in detail.
For instance, currently https://michielbdejong.solidcommunity.net/.well-known/openid-configuration announces "token_types_supported":["legacyPop","dpop"] so that's a strong hint that it may be a webid-oidc provider. But the only way for the client to know is to try.

When adding WebID's to an ACL document, the client will have no way to try out logging in as that user. It would be much easier if the profile document states whether or not its domain root is a webid-oidc-capable IDP.

Addressbook apps should not need to understand the details of how webid-oidc works, and should not need to change their business logic when the details of webid-oidc evolve.

So we may want to separate those concerns and let the profile card announce explicitly when this is true, so that clients have to do less leg work and get less false positives when trying to list which contacts have webid-oidc-capable WebID's.