Pluggable persistence

[parente]

Right now, we write notebooks / bundles to local disk when they are POSTed via the API. This means we can't scale the server horizontally nor can we operate in a PaaS/CaaS environment where the server disk is ephemeral.

Look into options available. Keep in mind we have to handle not only notebooks, but also any bundled frontend assets they require (e.g., zips).