Versioned paths

Within the guides application, there's currently no way to go back to older versions of Spree and find docs for that. We have http://edgeguides.spreecommerce.com which are for the *future* version of Spree, but nothing for the past. This PR aims to fix this.

Originally based off the 2-1-x branch, I'll be adding the 2.1.x guides to the path at /v2.1 and the 2.2.x guides to the path at /v2.2. By default, going to http://guides.spreecommerce.com will redirect you to the latest version, which is the first version specified in the version list inside of `Rules`. This means you'll end up on http://guides.spreecommerce.com/v2.2 when you go to the site, but then can go to http://guides.spreecommerce.com/v2.1 as you please.

I believe this will future proof the guides and will provide a much more stable URL structure for the future. The catch of doing all this though is that changes to guides may need to be applied across the same file in different versions of Spree. I think it's worth that small catch to get clarity within the URL.

Also worth noting is that the upgrade and release notes are *not* namespaced. These are relevant to all versions of Spree.