Warn when using HTTP instead of HTTPS

[RubenVerborgh]

See https://github.com/solid/profile-viewer-tutorial/issues/4

Cause found by @steven-borruso :

Found the "problem" ... my bad ... wasn't hitting my sample app site url with https ... was using http ... seems to be working ... can login now ... thx !

[james-martin-jd]

I ran into this issue again today, when using solid-auth-client. It looks like it needs to specifically be deployed to a server, accessed in Chrome, and not use ssl for this issue to occur. I was using the .Login function, not the popup.

It would be excellent if we could downgrade the error to a warning instead of blocking the action entirely with a JS error.