Add a `Blob` model that wraps `Gitlab::Git::Blob`

This allows us to take advantage of Rails' to_partial_path to render the correct partial based on the Blob type, rather than cluttering the view with conditionals.

It also allows (and will allow in the future) better encapsulation for Blob-related logic which makes sense for our Rails app but might not make as much sense for the core gitlab_git library, such as detecting if the blob is an SVG.