We may want to retain `http` until [Bug 796109](https://bugzilla.mozilla.org/show_bug.cgi?id=796109) is resolved, or all canonical URLs will be `https` and search engines will index them.

And, it's nice to have the same `SITE_URL` on dev in order to prevent pages on `www(-dev).allizom.org` from being [indexed by Google](https://www.google.com/search?q=site:www-dev.allizom.org+OR+site:www.allizom.org&filter=0). Once this PR is merged, we can remove `robots.txt` to make canonical URLs effective. `robots.txt` cannot avoid indexing.