phylesystem_commit_sha for each study

[mtholder]

It would be nice to have a property for each study (outside of the nexml element would be fine) that lists the last commit SHA that was created when the study was pulled from phylografter into phylesystem.

Knowing this would allow new content from phylografter to be placed in the correct spot in the revision history. Without this, I have to maintain a separate file with this data, and assure that the sync script is only run when it has access to that file.

A service to update phylesystem_commit_sha would be needed too. We would need to adopt a convention about the user who was allowed to call that method, so that we can assure that only the sync script updates the data.