Support canonicalizing URLs for SEO friendliness

From a slack discussion:

dpehrson [9:54 AM] @josh If you go that direction, make sure you include <link rel="canonical"/> tags on all those pages, and better yet force 301 redirects on each of those :show actions to ensure people get redirected to the canonical URL.

We need something that knows the canonical URL for a given controller action and redirects to it - that way if you access a category by ID, and there's a usable slug, we can redirect to the sluggy version. If we do that we don't need link rel="canonical"