Improve access to code tarball in the UI

[romain-intel]

Summary

Provide a simple button to download the code for a Metaflow run.

Motivation

The code for the run is typically available but hard to access. The path to the code package is visible in the UI but somewhat hidden and there is no simple way to download it.

Describe alternatives you've considered

None

Additional context

Related to https://github.com/Netflix/metaflow/issues/1009