Returns the URL of path on the home of user, an account from another instance, for GET /api/users/:key/home: their profile there by default, or another of its pages, such as /settings, where they change their password. A handle's host is its home's issuer, the API's origin, which needn't be its pages': those are where its /authorize page is, and the home's metadata says. While it doesn't answer, the issuer stands in.