Follow index redirect in discovery

[nielsvanvelzen]

Sometimes people use a baseurl and only have a redirect on the index page (e.g. jellyfin.local -> jellyfin.local/jellyfin). When the user inputs jellyfin.local we should send a HEAD request and see if there is any redirect. If the redirect is the same origin but with a subpath we should add that to the candidates.

This should allow easier setup in Jellyfin apps. Behavior can be tested with our own demo server which redirects to the /stable path.