3705: allow siteLogo parameter to include domain if served via https

This delta:

- continues to insist on the https scheme (explicitly or via scheme-relative or domain-absolute URIs)
- continues to forbid relative URIs
- allows specification of any domain

callahad says next step is to think carefully about security corner cases. Along those lines:

- we are relying on the browser to correctly sandbox the response as an image
- what information could leak to an arbitrary domain that doesn't leak from the authorization requests to the IdP's domain?