Fix favicons

The admin console shows favicons for federated domains.

For many domains, the favicons currently are not displayed, even though they're available. Messages like these are logged:

There seem to be at least two (potentially more) different issues. The jabber.de favicon is accessible only after following a redirect from HTTP to HTTPS. I'm unsure why the doomsong.co.uk favicon isn't being displayed (there does not seem to be a redirect).