HTTP proxy prompts for password despite being set in URI

An HTTP proxy of the form user:<email address hidden>:80 causes a dialog to show up prompting for the password. It doesn't appear to happen using socks5://foo:<email address hidden> - however we currently fully rely on SoupSession here and its behavior opaque, so it's hard to tell if this is consistent on all systems and between different proxy types.

We may need to look into implementing https://developer.gnome.org/gio/unstable/GProxyResolver.html#g-proxy-resolver-lookup (which may also be required for WebKit2).