Create Notebook repository cloning service, that adds the data staging layer to the repo

• Create "generatePostBuild" script that creates a postBuild (Binder) file that uses the script from #141 to download the user's data in a Binder environment
• Create async worker that creates a git fork (or repo structure) for a given Notebook based project, and extends its postBuild script to include the contents of the above

As a side task, we need to document whether Binder allows the creation of a Binder environment from a non-git HTTP directory