Use HTTP for RSS URLs

As per #7, there are issues accessing the site's RSS feed via HTTPS using Feedly (and anything else that doesn't play nicely with SNI certificates).

These changes force the HTTP protocol for the RSS feed link in the footer, and add in a `<link>` to the `<head>` pointing to the RSS feed (also using the HTTP protocol).

Even if we don't want to start forcing HTTP for the RSS feed link (despite the issue with Feedly), adding the `<link>` to `<head>` using a scheme-relative URL is still worth doing, and I can break that out in a separate PR.