Add a feature spec for users viewing cookbooks
Users can view basic information about a cookbook
Users can download cookbooks

Supermarket exposes a consistent URL scheme for a given cookbook, where
each download URL redirects the user to the URL of the cookbook
artifact. Supermarket tracks the total number of downloads both in the
aggregate and per-version.

Note that when a user downloads a cookbook without explicitly specifying
a version, we assume this user would like to download the latest
version.
Cookbook detail pages display the latest README

Supermarket extracts the README the same way it extracts metadata.json.
If there is no file in the tarball whose downcased name includes
'readme', it bails on the sharing request.

If the README's extension indicates the README is formatted as Markdown,
Supermarket converts that Markdown into HTML. Otherwise it assumes the
text is pre-formatted.
Show supported platforms on the cookbook detail page

Supported platforms are contained in the metadata.json file. Note that
the CookbookUpload::Metadata is now strict in its coercion. This is so
that Supermarket can catch and report errors with the metadata.json
"platforms" value format.

For now, SupportedPlatform records are directly associated to
CookbookVersion records. In the future, particularly when we want to
search and/or filter on the SupportedPlatform name attribute, we may end
up normalizing this records so that the relationship is indirect.
Replace get_version!('latest') with latest_cookbook_version