Implement cookbook download paths

This removes the cookbook_versions_helper and uses the routes for downloading a
cookbook. These routes redirect to the location of the tarball for downloading.
Knife follows redirects, meaning we can track the download count on each
specific cookbook version.
Update the cookbook show view to be more semantic.

This implements a joke of a method CookbookVersion#ghfmd_readme that should be replaced
with the actual readme content of a cookbook version that gets parsed with
GitHub Flavoured Markdown.