Improve visibility of Metaflow Code object

The code tarball saved by Metaflow is accessible with the client but it is not very easy to work with. We may want to make it more visible possibly even in the Metaflow UI.

Another idea would be to link it to the source control that the Metaflow code is in (if there is one). In particular, we could store the branch/commit information as well as a "dirty" patch file if needed. We can keep the same tarball to work internally but also linking it to the user's development environment may make it easier to work with.