Support webcal:// links
Calendar protocols are very confusing. Evolution supports CalDAV, which is an extension of WebDAV, which is itself an extension of HTTP. At first I thought webcal:// was WebDAV. The Wikipedia page says it is. Maybe it actually is. I don't know.
Point is, these StackOverflow answers say it's really plain HTTP. I've verified that webcal://www.gnome.org/start/unstable/schedule.ics works in GNOME Calendar if we simply rewrite the URI from webcal:// to https://. So let's just do that automatically, and save users the trouble of trying to figure out why GNOME Calendar doesn't like their webcal:// link.