Should we make this `null=True`? This will only exist after the repo has been cloned, but I'm thinking we may create instances of this model before triggering a cloning.

OTOH, `local_path` could be a property and a location of a repo working copy on local storage may be computed based on a convention and some setting. Also, we may want to think that file storage may not even be local (if we eventually want to make ployst load-balanceable).