Don't access Spree::Config[:site_url] at startup

Now `Spree::Config.site_url` is passed to `canonical_tag` only when the page is rendered. This also means that changes to the `site_url` in the admin are picked up without restarting the server.

Thanks @jumph4x, for releasing a new version of canonical-rails with this feature.

All settings in #3833 now have been fixed in master or in an open PR :tada: